Size is pretty irrelevant at the slot. You don't get press coverage there. You'd prefer a quicker guy at that spot because there's more space to work, you have a 3-way go, etc.
Plus a quick/fast guy can get to the Safety much faster. This gives the secondary problems.
It's good to have big WR's on the outside though. You generally get bigger Corners out there, more press coverage, there's less space to work with for a small/quick guy, etc. There's really only 2 type of routes you can run...vertical stemmed routes or inside routes...and it's good to have size on those routes in order to "box out" the CB.
Generally CB's are the fastest players on the field. (by NFL combine averages)
If you have a whole WR unit of small fast guys, there's a good chance that the defense can match-up with that. (they have small fast guys in the secondary)
However, it's hard to match-up with a 6'3"-6'5" WR on the outside, doesn't matter how fast your CB is. He's gonna get "boxed out" by that big WR.
I've been doing this for 15 years. Have faced tons of "fast" D1 level WR's down here. Completely shut-down two "fast" First Team All State WR's in 2019-2020. (just kept a Safety over the top) However, nobody has given me more problems than Western's Julian Lewis...cause he was so **** big and physical. We had no answer for him. Our CB's last season were simply too small. He would catch the ball even with our CB draped all over him. Same thing would happen in practice versus Bryce Gowdy a couple years ago. When facing fast WR's (Brian Robinson in 2019 for example) we just used our big CB to beat him up at the LOS and then kept a Safety over the top of him. But that doesn't really do **** when facing a big/physical WR who's 3+ inches taller than your CB... cause he's "bodying" your little CB on every short/intermediate route.
This is one of the reasons size/length matters at the CB position. (also longer reach/defensive radius)
Covering a small/quick WR with big CB's is easier than covering big WR's with small/average CB's.
Big WR's are always open, even when they're covered.
(referring mainly to the outside WR's)