I have to contradict @saj14saj here. I have frequently had bread made with underdeveloped gluten (my grandma uses AP flour and tends to knead very short, 2-3 minutes per hand, and use very short proofing times). The bread is soft and cakelike, but it has no trouble rising, and it is neither flat nor dense.
On the other hand, I have had bread with exactly the same symptoms as yours - first feeling great, then left out for a long time to proof. After that, it looks good, but one touch makes it collapse into itself. The reason was very clear: overproofing. There is no doubt that underdeveloped gluten cannot have been a factor in my case. First, I am experienced enough to know when my gluten is developed - the bread was kneaded well beyond windowpane test. Second, it was a large batch of dough. I baked the first loaf at the optimal time and it rose just fine and had a nice texture with traceable gluten sheets through the crumb. It was the second loaf, which I baked a few hours later (and the proofing loaf spent them in a 30 degrees celsius kitchen in summer) which made the trouble. The dough had exactly the "loose" feel you describe, unlike the normal, springy feel before the proofing. It collapsed on touch and would not rise at all. It also had a very strong yeast fermentation taste, unlike the other loaf.
From your description and my experience, my conclusion is that overproofing until your starter died in its own waste products is the most likely culprit. The simple answer would be to not let it sit out overnight. The right amount of time to let it sit would depend on the room temperature, on the amount of starter you used, but also its leavening strength, and that is a bit hard to judge for a newly created starter. My best suggestion is to use trial and error and maybe bake 4 hours after knocking instead of 8 next time, and the time after that adjusting with a smaller increment in the right direction depending on whether the bread turns out overproofed or underrisen.
I agree with many elements of the previous answers -- it could be due to the wet dough "resealing" and/or to the crust hardening too early and preventing further expansion. Doing a more horizontal slash than a vertical one is helpful to get good "ears," and extra moisture will keep the crust softer for a little longer to get more oven spring.
Frankly, although I did it for years, I don't find the plant sprayer method to do very much -- and if you're opening the oven periodically to spray in the first few minutes, you could be losing significant heat that could actually reduce oven spring. I concur with the steam pan method or using an enclosed pot. Note that pots don't have to be cast iron: any enclosed pot will significantly improve your crust as long as it is oven-safe at the baking temperature.
That said, I think this particular problem is difficult to troubleshoot without observing your specific loaves and slashing technique. Slashing deeper (whether vertically or horizontally) is definitely NOT always the answer and can actually deflate your loaves significantly if done incorrectly. With proper hydration, shaping, and oven steaming, it's very possible for quite shallow slashes to lead to great expansion. (As an aside, serrated knives are also about preference -- if you don't keep your straight-edge knives very sharp, serrated may be a better choice. However, they can leave your slashes jagged on the final loaf, which may not be as pretty and may result in unevenness for shallower cuts. I keep some of my straight-edge kitchen knives really sharp so they can be used for things like this.)
A lot of it depends on shaping and the stage of proofing your dough is in when you bake it, as well as the hydration. If you do a very thorough shaping (i.e., preshaping, bench rest, then very tight shaping of the final loaves), the outermost skin of the loaf may be very taut. Even a shallow slash could be enough to allow the loaves to open up significantly. If, on the other hand, you do a very gently shaping (little or no pre-shaping, trying very hard not to deflate the dough at all), the "skin" will not have the same characteristics and deeper slashes may be necessary. (As an example of this, you might consult the different advice given by Peter Reinhard and Jeffrey Hamelman -- the latter emphasizes detailed and tight shaping and thus advocates very shallow slashing; the former encourages gentle handling when shaping and advocates somewhat deeper cuts.)
The stage of proofing is also critical here: a loaf that is somewhat underproofed will likely have a more taut skin but will also hold its shape better even with deeper slashes. If the loaves are somewhat overproofed, they have a better chance of deflating or at least losing height with deeper slashing. The apparent moisture of the dough and its susceptibility to "reseal" the cut will also change depending on proofing stage.
Again, there are a lot of factors to consider. I would pay particular attention to the behavior of the loaves right after you slash them. Do the slashes spread significantly immediately (indicating a taut surface)? Or do they just remain close together (and thus risk re-sealing)? If you slash more deeply, does the loaf deflate? And if it does so, does it seem to reinflate in the oven, or do your permanently lose height? These observations can help troubleshoot the exact issue. A final concern -- sometimes if the dough is likely to reseal, waiting too long between slashing and getting the loaf into the oven can be a problem. A few seconds should not be an issue, but if you're taking a few minutes to slash and load a few loaves before getting them to the oven, that can be enough time for some cuts to close up again.
Best Answer
Smaller breads are usually called rolls or sometimes buns. There are also a lot of names for specific kinds of rolls, beyond obvious things like "sourdough rolls".
You can certainly take an existing recipe and just form in to more, smaller pieces, and reducing baking time. They'll look funny if you just cut it into pieces, though; you should reform them into something rounder.
It's hard to say exactly how much less baking time it'll take, since it depends on exactly how small you make them. It might be anywhere from roughly a quarter to half the original baking time, and especially for smaller rolls with shorter baking times, you'd probably want to increase the temperature so they'll still brown by the time they're done. Very small ones will be on the low end of the time range, and need more increase in temperature; larger ones will take longer and not need as much temperature increase.
Given all that, you might want to just look for a recipe for rolls that suits you. Simpler, less chance of messing up!