I've used both, actually a 15" and 10". The 15" is best, for sure. A 12" will probably work fine, but you'll need to hold it down tight against the bottom so it doesn't let grain around the sides (stuck sparge). I used a SS hose clamp on the pickup tube. My 15" doesn't require that. FYI - I went to the larger one because I needed the extra flow capacity to recirculate the wort.
Here are some pics for comparison.
