2g ~ 4g any larger can block the urethra and cause serious issues. I personally have a 00g and have no problems. I will be soon increasing to 7/16" and have seen pictures of 1/2". So at least 1/2"...
If you need to ask why a Prince Albert piercing then you don't need one. A Prince Albert piercing is an exotic male piercing, it adds sensation for both the wearer and the partner. Male exotic...