I've just kinda had this question in the back of my mind for a while and I wanted to see what ya'll think about it.

Whate pick sizes are the best for what?

Like are thicker picks easier or harder to strum, shredding, etc.

Thinner = sharper, clearer tone, less power, makes playing full chords easier though too thin and the pick will bend too much, slowing down the playing of individual notes.

Thicker = you can generate more output from really smacking the strings hard, but generally the tone won't be quite as clear. Less comfortable when playing full chords, but can be smoother and quicker when playing individual notes.

Ultimately there's no right or wrong way to go, some people shred with 0.40mm picks, some shred with 2.0mm picks. Some people can happily strum with 2.0mm picks, some people need 0.30mm.

Just try a bunch of different thicknesses and materials out, see what works best for you.
Yes, I know everything. No, I can't play worth a damn.
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I find that thicker strings are easier to shred on and they also sound better for it because they have more attack. Thinner strings are easier to strum on and they have a rounder tone.
typically, thinner picks are easier for shredding and faster playing, as the lack of thickness helps them move faster from string to string with less resistance. But they feel like paper and I've always been paranoid that they'll break on me at any second.

You don't to use too thick, or else it'll get caught and slow you down. I use a 1.14mm pick, but I file it down to a smooth edge so i get the stability and grip of a thick pick with the added bonus of having a smooth, thinner edge for playing.
I can sweep much easier w/ my jazz III's than any other pick...
but mostly it's just preference; there's no be all-end all of guitar picks...
It also depends on the type too. I find that a thick Dunlop stubby pick will be sound more similar to a thinner pick than a thicker pick. It actually has characteristics of both.