Your body will tell you how long is too long. If you can't or won't listen to your body then you'd better not play around up there (or at least keep it to 5" or less).

Personally I've taken up to a bit over 12" inserted (all but the largest "bubble" on the Cowboy link). As far as the experience goes, I find that things change when you get past the rectum (a toy over about 7"-7.5" long), and play becomes slower and less obviously sexual. It's intense, but in a different way. When I'm interested in fast-thrusting high-power buttplay I go for something in the 6-7" insertable range. It hits almost all my buttons and isn't so slow and "reflective" as the big toys.