She looks pretty normal in shape. Not specifically needing an Arab saddle... just pretty normal. My horse is an Anglo, but he's more TB type than Arab so my experience isn't really any help.
Usually, you take the gullet measurement 2 finger widths behind the shoulderblade, with the horse standing square on a solid surface, and the back shape from around that spot back to the last rib.
Being an Arab she will more than likely have a short back, so you will need a fairly compact saddle. Make sure it's the size that fits you, so that you can ride effectively, but it cannot be in contact with her back past her last rib. If you HAVE to have a big saddle, and her back is too short for one, you could try a saddle without the bulgy bits at the back...there is a word for them but I can't for the life of me remember what it is!
Fit the horse for the shape she is, not the breed. Tell any saddle salesperson in any store that your horse is a TB, for example, and they will immediately try to sell you a real narrow saddle. Arabs have a reputation for being real narrow where I am, but the majority actually aren't, so watch out for that.
I think your girl is probably a medium wide to a wide, comparing her to my boy's shape. Monty is fairly similar in width across the wither. Having a saddle that is too narrow may be contributing to her lack of muscle buildup - as soon as I swapped Monty to one of my saddles from the one he came with, he put on SO much muscle. He went from a medium to a wide in two weeks' time.
A CLEAN SLATE FOR THE FUTURE