Are you wanting the dimensions of the components (display and circuit board) or of the lit display area? I would highly recommend not cutting out your slots until after you have your lights. Once you have them, take a piece of clear plastic (heck, wax paper would probably work) and trace the display area. Once you have that, transfer it over to your armor and carefully start trimming, occassionally putting the lights in making sure you are going overboard. Making the cuts just using posted dimensions can be risking and difficult to fix if you cut more than you wanted to. If you weren't planning on using Shackman's display, then here are the estimated measurements for the slots in the chest we came up with based off the MoM suit:
Top slot is 1/8" tall
Bottom slot is 1/4" tall
Slots are 1/4" apart.
Slots are same length; 1 5/8" long