You might have given the answer in your question. If the device is to be used as a steamer as well, then there has to be enough room at the bottom of the pot for steaming. How much is enough? There are two things to consider:
The bottom section should hold enough liquid that there won't be concerns about boiling the pot dry for anything you might want to steam. Longer steam items might include mussels, lobster, or tamales.
There should be some 'air margin' for foods like tamales or broccoli, so that the boil splatter doesn't render the food soggy.
Based on FuzzyChef's comment, it sounds like there are tools that suit your needs better. Perhaps it's time to put your steamer on Craigslist? :)
You need 1.1x as much water as pasta for al dente! I measured this myself, cooking penne rigate (in water, not sauce) - 200g of pasta weighed 420g after being cooked and thoroughly drained.
The estimate below from the nutrition facts is 1.4x, which probably corresponds to typical American overcooked pasta - a surprisingly large difference from mine. I'm guessing you'll be somewhere between, maybe 1.25x.
You can also ballpark it from nutrition facts. Cooked spaghetti has 31g carbs per 100g pasta, and dry spaghetti has 75g carbs per 100g pasta. So 100g dry pasta turns into 100g*75/31 = 242g of cooked pasta, meaning the added water was ~1.4x the weight of the pasta. So for a pound of pasta, that's 1.4 pounds or about 2 2/3 cups of water. Given people's tastes, this might be a little past al dente, so I would personally start with maybe 1.25 cups of water then add a little more if necessary. (I'll also try cooking pasta normally and weighing it if I get a chance.)
Note: if you have a significantly different variety of pasta, it will obviously behave differently. This is for pasta that's 13% protein and 75% carbohydrates. For the standard 2oz/56g (dry) serving on the package, it'll say 7g protein and 41-42g carbohydrates. I checked Barilla, De Cecco, Ronzonni, Garofalo, Safeway store brand, Trader Joe's store brand, and those nutrition facts, and they all matched. If you're branching out to other styles like whole grains or egg noodles, things will obviously change, but things are very uniform in the US.
Best Answer
The pot choice should depend on the amount of water (which depends on the amount of pasta you're cooking) rather than the other way around. For detailed advice on amounts, see these answers (answer 1, answer 2).
Generally (not using fresh pasta or very long pasta when more water is necessary) you just need to cover the pasta. A bit of oil on top will help keep it from boiling over, as will keeping the lid off after it's begun boiling and just simmering it rather than a rolling boil.
With longer pastas like spaghetti you should add more water than just enough to cover the pasta, but as long as the top of the water is more than an inch below the top of the pot (after adding the pasta) you shouldn't have trouble if you follow the precautions above.