[toast0]

Some limits are technical (and in that case the hard limit is often bytes, but sometimes code units or code points, or broken if you told MySQL utf8 instead of bytes or utf8mb4), but in many cases, the limits are for aesthetic purposes: a post title or a username often is often required to be fairly short to look nice; in an ascii or latin1 world, those limits are usually expressed in terms of characters, but graphemes might be the right thing to limit in a unicode world.

[kmill]

"Your username must be 1-4cm when printed with 12pt Times New Roman."

I kind of like the idea of minimum length in cm as a password requirement.

[martin-adams]

What about "Your username must be no longer than 3 seconds when spoke out loud"

or, "Your username must not take more than 0.001ml of ink when printed at 12pt"