what dimensions for a 12” sb cr77 subwoofer?

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.

if you can have the driver measured, get complete specs, designing a good box will be fairly straight forward

To make best use of the 'book about building speakers', you will need the TS parameters of the driver in question.

Online caclulators or quick downloads will guide you to proper dimensions.

With good woodworking skills, the TS parameters and some experience, you can get remarkable performance from a DIY sub.

The plate amp makes a difference here, so don't skimp.

Parts Express or Madisound may help with driver parameters. They maintain a database.