......... a little explanation and the right choice of pocket size is easy.
Stud work dimensions have changed a lot since the days of 4" and 5" timber. Generally available stud work now comes in various sizes - 63mm CLS, 70 steel, 75mm scant, 89mm CLS and 92mm Steel, so how do we match our pocket thickness to the stud you are using?
Using 63mm CLS?
Choose our 70mm frame
Add one layer of 6mm plasterboard to one side of your studword
Add one layer of 12.5 plasterboard each side of both the pocket and stud work