My few cents...
First the harsh unpleasant truths:
-Length preferences are highly personal. While some of the benefits and downsides of going up or down in length are somewhat objective, only you can decide what you actually prefer.
-There is no easy shortcut here... the only way to figure this out for yourself is to actually try stuff... usually for a longer time.
To complicate matters further.
-There are a lot of factors that influence your perception of how a knife feels and that will make it feel longer, shorter, or easier / harder to wield. For example total weight, weight distribution / balance, height, taper, all have a significant impact. I have 270s that feel easier to wield than my 230 Wüsthof simply because they're lighter and balanced differently. Knives can 'feel' significantly longer or shorter than they really are.
-Then there's personal factors coming into play; your grip and cutting style. As others have mentioned, pinchgripping somewhat 'shortens' the knife. For rockchopping, more length generally improves the ergonomics, etc.
-Knife makers are not consistent in how they measure length. So to compare 2 extreme examples; my Masamoto KS 240 (which runs significantly long at 255) is almost exactly the same length as my Ashi Ginga 270 (which runs significantly short at around 260).
-Over time your skills and preferences are likely to change. This could go either way... but for me things started feeling 'smaller' over time. A knife that feels large now might feel wimpy a year from now. You might also change cutting techniques. So your own preferences are unlikely to remain completely static.
The more objective upsides/downsides:
-Shorter knives will generally be 'easier' to use from the start since everything is just extending less far from your hand.
-For the same reason it's a bit easier to use the tip accurately on a shorter knife; a longer knife exaggerates the movement a bit. But this can be mitigated; you get used to more length over time, and there's always the option of crimping up on the blade a bit and/or putting your index finger on top of the spine. Or just relying on your guide hand.
-Shorter knives are lighter for a given grind / geometry / height. Though whether or not this is a good or a bad thing is a personal preference thing.
-Shorter knives are a bit more nimble. For this reason you usually see even the most hardcore 270 gyuto enthusiasts breaking out honesukis and petties when they're doing boning work or trimming meats.
-Board / kitchen size. If for whatever reason you're really restricted in available space / board size (they usually go hand in hand; people get small boards because they just don't have the space in the kitchen) then larger knives can become problematic. Although one shouldn't overstate this. I don't really have a problem using a 270 on a board that's just 35 cm deep. Or a 240 on a 25 cm deep board. But you do want to make sure you're not poking into any bowls on the other side of it...
-Longer knives are outright more efficient because you can put more product under the knife. It also makes it easier to cut piles of anything, for example when you're doing a brunoise.
-Longer knives make it easier to cut larger and taller products.
-Longer knives are more ergonomical if you don't cut straight down (so rockchopping or guillotine & glide), because the angle the knife makes in relation to the board becomes lower.
-Longer knives make better slicers (less back & forth sawing).
-Longer knives give the maker more room to work with, so it's easier to make a useful profile.
I probably forgot some things. Personally I started off with some 200 stuff...after long deliberation went up to 240 with the same doubts as you have... then after a while bought some 210...and at this point I'm exploring 270s. In the end it's worth having stuff in different lengths. The 270s are great when I'm making a salad with big bulbs of fennel, but when you're just cutting a shallot and a few cloves of garlic it doesn't give any added benefit.
