Dave,

All it means is, if it was worn by Canseco it would have to be before September 11th. After September 11th they wore the US flag on the back to show respect for the 9/11 victims.

I linked you 2 jerseys from 2001 showing the jersey size. If he wore size 48 i would think it would be consistent throughout his uniform sizes that year. There are exceptions i have seen though.