How to measure western tack for a horse?

Ok I have a horse that is just coming back from the trainer, I asked him about what size of saddle he uses he just said his usual saddle, do i get a saddle that fits me or my horse or both, how do you measure for a briddle, does it change with different bits, umm, how do you measure for a collar and cinch etc. Is there any other tack neccessary for barrel racing / rodeo thanks anything that can help me with this question would be awesome, oh and i will probably buying the tack usd so i might end up not at a tack storre thanx