You only have a FEW questions? Stick around, you'll come up with a few dozen more.
For an example, my lilliput 629 touchscreen case measures about 7-1/2" wide, 5-1/4" high, and 1-1/4" deep. If you take the display out obviously you'd subtract an inch or more on w and h.
I don't have mine mounte in dash (see my pics), so check out the FABRICATION forum for some details about how others did in-dash installs. i almost was ready to mount mine in dash, but I am considering a 10.4" screen 'cause I'm older and blinder than most...
I'll leave it to the guys that have done dash installs to inform you more. Welcome to the obsession!