Let's say for example the magic items are as follows:
Torvald's Insecurity. Your size is increased by one for as long as you are attuned to this amulet and it is on your person.
Smeek's Sleuthing Stone. Your size is decreased by one for as long as you are attuned to this stone and it is on your person.
Would this be balanced? The opportunity cost of an attunement slot should even it out if the effect was on the more overpowered side.
For clarity, I'm a player, but my DM is amenable to the idea of homebrew items, feats, races, classes, etc. So, I was asking for the sake of understanding how much impact certain abilities would have on a magic item's power to inform any future crafting endeavors, like splitting Enlarge/Reduce into two separate items of permanent effect. I would hope to eventually make a system that helps with the analysis of homebrew magic items, like Detect Balance by Eleazzaar, for future reference.
Best Answer
Not overpowered
Torvald's Insecurity
The most common effects of being larger include
Note that the feature of the enlarge spell that you deal +1d4 damage is a feature of the spell. Just being larger by the game rules does not give you more Strength, nor does it allow you to use oversized weapons that deal more damage (the rule for that is a DM facing rule for designing monsters). So the item that makes you larger would give you the list of benefits above. I think this might be exploitable in edge/combo cases, for example with a class or race that allows you to already become large, but overall, the above things seem not overpowered to me if you have to give up an attunement slot for them. After all, you could get these very same benefits from a simple, uncommon potion of growth for several hours.
Smeek's Sleuthing Stone
The same reasoning applies for the item that makes you smaller. In fact, there
Again, there is no mechanical penalty on Strength, or benefit to Dexterity, and there is no direct effect in the rules on damage, movement speed, or even Stealth proficiency or advantage. So this if anything is rather weak, most of these effects are rather detrimental. You would need to add something like granting a bonus to Stealth to the item, if you wanted that. Unless I have a very special build that has other ways to benefit from being tiny, for example, I would not spend an attunement slot on this item if I had other reasonable options.