[Burl]

I have some boxcar sides I cast that are about 24" long, so 3 feet is probably possible. These are ambient castings though, not pressure castings. The trick is to make the pattern with shallow detail. And when you pour it, be quick! Any air bubbles on the back get filled with CA & sanded down smooth on the belt sander.

http://burlrice.com/_LS_60ftHiCube/index.php

The best advice I can give you right now is: start small & work your way up. Trying a 3' casting right off the bat would probably have frustrated me to the point of giving up.

 

[Burl]

Sorry for getting off on a tangent...

Personally, I have a 2.5 gallon Sears paint pot that I use for pressure casting. I made my own vacuum chamber from a 6" PVC coupling and lexan (never use Plexiglas). It uses a small vacuum pump that I got off eBay - I think it's a Gast pump - and it only draws about 25-26" vacuum. If I had it to do over, I'd get a bigger vacuum pump, and make a bigger vacuum chamber.

I only degas my RTV long enough for it to rise & fall. Then I'm careful to pour with a long, thin stream and that seems to get all the bubbles.