I buy nothing but 1-1/4 for bricks/blocks
and 3/4 for everything else. Lambswool/sheepskin.
I always clean my roller sleeves even if I use them in oil base.
(remember I'm old and come from a mostly forgotten oil based paint trade)
As they wear down the 1-1/4 becomes a 3/4 then 1/2 then finally a really good finish enamel roller, then it finally gets it's retirement party because it has made me some $$
Except for buying them I don't really go by measurements, more by look and feel. I do use the six inch long handle 'slim jim' rollers....
those things rock!