how to know what size girth I need?

My horse uses a 34 inch western cinch.

What size english do I need?

Thanks

Depends on the length of the billets of tbe saddle youll be using. For example, my horse is a 46 in a jump saddle but a 20 in a dressage saddle.

If you're working with a trainer, learning the tack of a new discipline should be part of your lessons.
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You have to measure. You can't convert from your Western size to an English size.

Place the saddle, with pad, on the horse in the correct position.

Measure from the 2nd set of holes on the offside to the 2nd set of holes on the nearside. It's best if someone is pushing down on the saddle while you measure. If no one is pushing down, you may want to measure from the 2nd set of holes to the first set. When the saddle if fully girthed up and tightened, you want to be using right around the 3rd set of holes.

If you don't have a flexible tape to measure with, use a piece of string, then measure the length of string when you're done.