How did we come to say "size up" for when you're checking out someone? I was just listening to that camp song "The Other Day I Met a Bear" and it has that phrase in it . . .

He looked at me, I looked at him,
He sized up me, I sized up him . . .

As far as the etymology, I checked my 2 favorite sites, world wide words (Quinion) and word detective, but it wasn't there. However, it seems to make sense to me. In order to check someone's size out, you have to take a close look at him. So, you are 'sizing him up.' I don't know, though.

size v. ... 5. colloq. (orig. U.S.) Usu. with up: To take the size or measure of; to regard so as to form an opinion of; to make an estimate of 1884.

Aput, the usage you cite is noted as colloquial. But can't size be used as a verb non-colloquially, as in "to size a suit of clothes properly"?

Earlier senses in the OED are 3. to make, give, or adjust the size of something; and 4. to arrange or line up by size.

I think 5. is really about size up, which begins with a literal sense 'take the size of', and what the 'usu.' is doing in there is noting that occasionally the colloquial sense 'form estimate' can be plain size, but is usually size up. Also, my copy is an old edition and we might not think of it as colloq. now.