How does different flap lengths on a dressage saddle affect the horse and rider?

I am a pretty short rider, and the saddle I am thinking of buying offers extended or shorten flap lengths, but it is quite expensive as it is considered a "custom" saddle when you order this. I ride a section c welsh pony, so she is rather small as well. Do I need to get a smaller flap to be comfortable? I have only ridden in jump saddles before so I don't know much about this. Any suggestions would be greatly appreciated!