It partially depends on how snug you prefer to be in your saddle also. My saddle is a 14" barrel saddle (I'm about your weight, but taller), but I have a friend that is the same size and rides in a 17" roping saddle.
Granted, you'll want a snugger saddle while doing barrels, but if you're comfortable in a larger saddle then use one. I also know you usually ride english and as a general rule, western saddles are around 2" smaller than english. As in, if you normally ride in a 17" English saddle, a 15" Western saddle will be close to the same size seat.