I'm sure this has been asked a lot but I'm curious what size saddle everyone rides and what you guys think I should be riding in! A WP trainer and I got into an argument that I ride too small a saddle. I'm actually looking to buy smaller!

I'm 4ft 11inch, 109ish lbs and a size zero-2 usually. Currently ride a 14.5 and feel like I'm slidding all the way back in my saddle out of turns and catching his face. He has so much power.

With the way you described yourself, I think a 14.5 is pretty big for you, I'm 5/7 and 159 lbs and I ride in a 14.5 to a 15 depends what brand saddle, I think you would be in a 13.5, just depends on what you are comfortable in. WP and Barrel racers ride way different then each other, its a whole different ball game. 

I have a 14" that I prefer to ride my younger horses/colts in, its my favourite saddle. When I rode with a working cow horse trainer he always told me that your seat should be like a third stirrup, and you can shift your weigh to help your horse accordingly. I know during a run that you don't want to be flinging yourself around, but when you are riding younger horses it can be nice to move around a little bit more to help them catch their lead, make it easier for them to come back under themselves from a turn, teaching to back up, etc. I ride a 13.5" on my open horse that has a solid foundation and doesn't need any 'help' with leads etc other than the occasional tip on hip to get up a gear.

Stirrup length is also another important factor. My younger gelding is quite hip high this year so I lengthened my stirrups to stop from getting so tipped forward when I ride him. That took some time to figure out what was going on lol. Normally i ride in a little bit shorter stirrups in most opinion ( I am almost 6ft tall ) but in this circumstance having a notch longer has really helped me keep my seat more stable to help my horse.

All in all I don't think its 100% about seat size, although it does seem like you could be in something a little big for what you described. But there are benefits to both, depending on what you are riding. Other disciplines have very different opinions too. Also depends on what your actual seat is like, if you have a flatter seated saddle, you can generally go up a half inch. Using Double J vs Pozzi for an example. Pozzi's are more dished and require you to size up a half size typically. I would ride a 13.5 on the reg double j, a 14 in the pozzi.  

I'm 5'7" and 145lbs and I ride in a 13.5" saddle.

I'm 5'3 and about 125 pounds and ride a deep 14" with some breathing room, I can ride a 13.5" comfortably. IMO you should probably be in a 13-13.5 saddle depending on the brand. 

Barrels and WP are two very different discplines, what works for them may not work for us. There isn't speed and hard turns in WP. If you feel like a smaller saddle would keep you positioned better, get a smaller saddle. 

First of all if your saddle works for you don't change. It is not too small. My trainer is about as big around as my little finger and all her saddles are 14 inch. I'm 5'5" and weigh about 160. My saddle is a 16". (Way too big for you but making a point). We went to the World Bucksin Show and did not have time to change between my classes and hers. No way I could ride in her saddle so she had to ride in mine. I laughed and told her it looked like she was sitting in a bathtub!  Western pleasure has a whole different seat from our sport and they are judged on that. We aren't. Will try to attach a picture of my trainer in my saddle but internet is slow so it may be a no go!