Why are there so many different screw sizes?!

This is a belated answer so hopefully it can be of use to folks who search for and find this question.

Ahh...the infamous screw chart question; this is the bane of every DIY iPhone tech's existence :>). Apple engineers thought it wise to have pretty much every screw be different from the other. I can imagine them having lulz at the thought of finding new ways to make repairs difficult.

At any rate, there is no easy chart and no one sells them un-jumbled. You need to go through the appropriate guides to find the screw sizes. The truly critical screws are the ones that screw into standoffs on the logic board. If you put a longer one, even 1/10th of a mm, then you can seriously damage the board. The ones that screw into the housing or standoff screws are less critical.

If your caliper is giving you readings that are off by 0.05 mm and repeated zero'ing of the caliper doesn't make it change, I would always err on the side of caution and take one slight shorter, especially for the ones that screw into the board.