How much shrinkage depends on the moisture content of the wood when you bought it, and the normal moisture content of wood in your area after it dries. As a point of reference, hardwood typically shrinks about 2 - 5 percent going from wet to dry, with most of the shrinkage across the grain, and only a small amount lengthwise. You can get a better idea of normal shrinkage from this website http://www.popularwoodworking.com/tricks/how-to-calculate-wood-shrinkage-and-expansion

Unfortunately PT lumber is often very wet, up to 50% moisture content, so the shrinkage can be well beyond normal tables. Unless you have a moisture meter, you are not going to be able to tell how wet your boards are. If you do know, you can use the actual moisture content in the website I linked to calculate the actual shrinkage.

 

I don't have a moisture meter but I prefer the method in this link as I feel it's more accurate as it gets right down to bones, even using smaller samples than recommended.

http://www.deldot.gov/information/pubs_forms/manuals/mat_research/pdfs/doh_2.pdf

 

I was told that 'local' lumber yards have kiln-dried PT lumber that doesn't shrink. Not sure if that's true anymore, but maybe you could check into it if you have a local lumber yard.