How do these socks stand up to bar knurling rub?
They are pretty durable, but will fray over time.
Can you wear these at a USAPL Meet?
You'd have to check the USAPL rulebook. Usually we would say "Probably", however the USAPL/IPF may have specific rules about equipment with branding on it, like our socks.
Is it only one size?
That is correct, they are one size fits all
I’m looking for a good sock to squat and deadlift in without shoes. I know I can use any old sock but thought it would be wise to have a dedicated sock. The gym doesn’t allow bare feet. Can these socks be used “barefoot” for squats as well as deadlifts?
Yes, these socks are often used for 'barefoot' squatting and deadlifting.