If you need more support to keep from sliding forward, then rather than moving back (where the body of the bike gets bigger) you should firm up the front of the seat with a higher density foam. When making a change like this - I always try to do it in a way that I can put it back if it doesn't help.
Get yourself some higher density foam (I actually go in to a local distributor and describe what I want).
Start out by just adding say a 1/2 inch layer of foam on top - that does it for some people. If you need more - then cut off the front 6 inches of your existing foam - straight down so you can put it back if you want. Cut the new piece to replace it and fit and shape it (I use a product called Dragon Skin). Drape the cover back on and cover with plastic so you can duck tape around it (so it doesn't fall apart). Check it an see if it is to your liking. If it is now too firm then the easiest thing to do is to slice it from the front to the back - going down at about a 30 degree angle towards the back of the seat, then do the same to the original piece of your seat - on put the slice of the original on top of the heavier density foam.
With only 2 cuts you can easily glue the foam back together if you want and put the seat back to stock. If you can't get it the way you want - then take it to an upholstery shop and pay them to do it. Bring the bike so you can sit on it and explain what you want.
If you aren't trying to keep from sliding foward then for us to help you probably need to explain why you are 5'2" but want to sit further back.
