So I have a 7-string Schecter with a fixed strat-type bridge, and I'm going to install a Floyd Rose bridge on it. the guitar has no pervious body routing for a trem bridge. The bridge I bought is from Warmoth. http://www.warmoth.com/7-String-Floyd-Tremolo-Chrome-P542.aspx

My question to you is this: How long should the cutout in the back of the body be? How much do the strings need to be streatched out? I'd like to have this bridge set up to be floating.
length of spring cavities should be around the same length i guess. because the size of the springs are the same as 6 string floyd's.

from my guitars. they all seem to be around 125mm long (from front to the back of the spring block cavity)

for the width. id measure the width of the spring block add 5-6mm (3mm each side)