I think the one I have is 20"x20"x4". It has a bunch of knockouts around the perimeter of the box.
I detailed this in another thread, but here is how you slowly incorporate a new service panel with new wiring:
1. Place you panel and J-boxes - Make sure you are observing POCO and local permit rules (I had to show that the POCO had placed a sticker saying it was OK to place a service panel where I did) You will need to talk with your POCO planning group and your local BI.
2. Take the biggest breaker in your system that is relatively lightly used (stove OK, Air conditioner not so much) and remove the circuit that is on it. Run that existing circuit over to the new panel and place it in the same amp breaker. Run a new circuit back to the original panel, also with that same size breaker, so you are connected with a breaker on each side that is the same size. You are now powering your new panel from the old panel via this circuit.
3. Cut over existing circuits from old to new at your leisure. I ran my whole house off a 40 amp breaker for months while I did this.