Latex do lose some size as the latex cures. Normally I only do a snap mold if it's a really the only way I have of doing something or I'm too tapped out to buy a good tin sil or platinum silicone. Preferrably I'd go for a silicone.....Maybe a gel10 would be better than a latex mold for you. You can't really go wrong with platgel 10 as it's a 1:1 mixing ratio by weight.
Anyway. When sculpting when you will be molding in latex, factor in at least 10% for shrinkage <you can get between 10-!5%>. Place whatever object you are molding on smmoth flat surface. Like a nice piece of acrylic. Hopefully what you're sculpting will have a flat bottom. Once secured to the acrylic seal the sculpt with an acylic sealer like Krystal Klear, at least 2-3 coats and spray again with a dulling spray, this will stop the liquid latex beading up on the surface. Paint your latex on in layers waiting for each layer to dry completley before the application of another. You can force the drying by using a hairdryer. Do at least 6-8 layers with thin latex as this way you'll capture all the details. Then you can do a few layeers with a thicker batch thickened with a thixotropic agent you can get from the same place you ordered your latex. You can even add layers of nylon <as in tights> in between these final layers. As the fabric will creat a bond between the layers of latex and increase the mold strength and ultimately the life of the mold. But your mold will deteriorate especially when using resin. Once the mold is done. Don't peel it off just yet. As you need a jacket to go with it. Make A box wall to surround the mold and then mix up some plaster of paris or if you have the time you can use fibre glass. Pour the plaster in the box and wait for it to cure.then you have a mold and a support jacket for it too. If you go the fibre glass route you don't need to make the box wall, just spray release on the outside of the mold and simply lay your mat on your mold and wet it out with the resin. Do 2 or three layers and you're good to go.
You could always go the waste mold route which is how I do all my pieces that need a fine polished finish. Make a plaster mold of your piece, clean the mold. Spray the interior with grey primer and then add a wax release to it once dried. Then you're ready to fibreglaas. Once the piece is finfished and the fibre glass cured fully. You smash the mold carefully. Sounds madness but it's worth the hassel, as you're left with a pice that can be sanded and filled and refined until you're ready to mold in silicone.
Hope this helps explain the latex molding process and the other bits help you decide whether it's worth it or not. As I said Silicone maybe more expensive but it really does provide you witha greater quality of mold.