I have read printed materials concerning the volume of the Glove box (500 cubic inches rings a bell but don't hold me to that), which I assume would include both the upper shelf and lower area. I've looked around but can't find the reference... maybe someone else can help. Thought is that if you can find the volume number, you could calculate the volume of the upper shelf easy enough and subtract that from the total to get the volume of the lower.
Hope your electronics project can be confined to the shelf space alone, however. Many members have complained that while they are willing to sacrifice the upper shelf for the CD changer, they wished the lower was larger, presumably for standard Glovebox items. I'm not sure that many would devote both areas for any single use.
BTW, I like your avatar!