what dimensions for a 12” sb cr77 subwoofer?

If it were that easy, everyone would make their own. Companies put years of R & D into developing these type of things. Theres no way that you will be able to build anything that even comes close to a professional design. Something tells me you already suspect this. If you just want box dimensions, look at some subs with 12' drivers in them and go by that.

There are books about buildingspeaker enclosures..
Suggest getting one of those. There are ideal dimensions and whether to port or not, size of the port, length of the port tube..
All sorts of stuff based on the ideal 'Q' of the particular speaker being used.
For a starter try Madisound site. They sell kits and speaker parts. And a lot of info on do it yourself.

http://www.madisound.com/

Drivers are designed for either ported enclosures or sealed enclosures. You should start by finding out which driver you have.