Lol, I worded it wrong.  I am using the depth jumps with max effort to take advantage of its shock stimulus/plyo training, but I just meant since the volume is low, it provides a good warm up for the rest of my work out as well.  
-I'm assuming if this is the case, then go with the 3x5?
-So as long as I am keeping potential overuse issues in check, I can gradually move up to 30 inches?  If so, do I just keep moving up height whenever I am consistently getting higher on the current box height?
Edit:  Also, the room has mirrors on every inch of every wall so I don't think touching the wall will work lol