If you position and stitch the zip from the closed end, (It's normal to stitch a zip from the open end first) you can get away with a shorter zip as you can stop stitching when the foot reaches the zip pull- further along than when trying to avoid the zip pull at the start. Jan
I think it depends on what the size of the purse is. Which one did you have in mind?
www.wawak.com for zippers. The buy2,get 1 free offer is over but the prices are still the best. I got 9 inch ones that came to 37 Cents each including postage.
I would go 2 inches over the size of the purse (the direction the zipper going). So if the purse is 7" wide use a 9" zipper. I have been buying my zippers in bulk at 14" so I have it available fro anything up to my 12" hoop.
In several ITH projects that I have done I used 7". Thats what I purchased in almost every color that they had. Hugs & Stitches, Liz