The "pick n pluck" foam on the inside cuts in 1/2" increments so you want make sure you the case is at least an extra inch wider/longer than what you are encasing.
What I did when buying mine was to draw on a poster board or piece of paper the inside dimensions of the case and place whatever was going to go inside to see how it would all fit.
There is also a little bit of foam at the top of the case in the lid, so give the vertical dimensions a bit of play also.