You are right, extremely different lengths.
Earlier the long ones would be considered beginnerboards and raceboards and lightwind boards.
But with the bigger wings we have today, many only have short boards, and can ride in everything but marginal winds now
It is all personal preference, some like boards as small as possible, and quite flat (they have to be if short), meaning lousy if they touch down on the water, but if you keep flying most of the time it does not matter much.
Others like the extreme nose scoop/rocker, a tad longer boards but still short 115-120 cm, as they behave like the shortest in terms of agility, and have the best touch down and fast start characteristics.
And in some conditions longer boards make sense, I use 130 cm most of the days, a perfect allround size for my 78 kg, and 150 cm big volume for marginal winds, makes a huge difference.
But no consensus about this whatsoever, so I would just get a "medium" length not too flat board now, if you are average weight, and start there
The shortest ones definitely sink when lulls, as opposed to bigger boards where you can ride on and wait for the gust or wave to get up.
And they dont start just as early, loads of drag when small.
If you are really talented and keep foiling it doesnt matter - but I havent seen (m)any being able to do this in real life