and why is Toolson too small to be a wing? He is 6'5".

BYU next year has 5 wings:

Johnson- 6'5" Senior
Robinson- 6'7" Junior
Knell- 6'5" Junior
Saunders- 6'5" Sophomore
Toolson - 6'5" Freshman

4 of those have been a part of BYU's rotation in the past with 3 of them playing significant minutes last year. Further, I think whomever ends up being BYU's back up PG, Stewart or some unnamed portal transfer, will probably play with Hall for some minutes each game and I don't know who will play PG and SG in those alignments but Hall also has adequate size to defend most college wings as he is 6'4".

I do think Wahlen will likely play the 4 at BYU.