What ban are you speaking of?
Generally, the ban being referred to is the Magazine Capacity 10 round limit imposed by the 1994 Crime Bill ( which was a crime in and of itself ).
The Ban expired in 2004 and is no longer relevant except when dealing with the 10 round Magazines built during that time.
Also, some USA States have Magazine Capacity restrictions, usually 10 rounds.
I purchased my witness .45 compact in 1997. does that make it a pre or post ban?
Post-ban
Also I was wondering if the full sized magazine will work in a compact. If so, is there a sleeve or something like that to protect what extends beyond the bottom of the grip?
Yes, a full size 10 round .45 Magazine should work in your Pistol, but I am unaware of any space filler sleeve made to adapt it to a Compact.