Dimensions, I used 92"x48", and I haven't found 48" to be too wide. I did 92" instead of 96" siomply because I cut the fabric too short :P
The problem with carpet underlay, as was mentioned before is that is is an inconsistant surface. Go with a simply 1/2" closed cell foam. It may cost you up to about $30US to cover an 8x4 table.
For the fabric, I have never heard of the leather idea, which is interesting, and very old school. For my first table I went to a pool table shop, and they sold me a used pool table covering cheap. That stuff is very durable, which is good for a home table.
For my second table I used velveteen, which is ok, but not ideal. Its best selling point is that is is cheap cheap cheap. The problem is that the threads are directional, so it kind of has a smooth side and a, well, less smooth side. I certainly wouldn't say rough.
For my next table I am going to splurge and buy speed cloth.
I made my table with a wooden racetrack (similar to the one in the post above) and had the cupholders recessed in the wood. I like this style as an option, but I have decided I want my next one to be playing surface all the way across, with rail mounted on top. I like cupholders for the table, so I will get the "slide under rail" style.
Chip trays are only important if you are planning on having a cash game with a dedicated dealer, and the drop box is only necessary if you are planning on taking a rake from said cash game with a dedicated dealer. If that is something you are planning on doing often, go ahead! But otherwise save your money (to invest in a better fabric or something)
The only reason I could see for you to need additional support is if you are using mdf or plywood that is less than 3/4" thick (not recommended). However, if you are using folding legs, this support beam of yours will get in the way.
If you look on line (
www.hoepokertourney.com) you can find a lot of plans for great tables, and most of the ones that I have seen that have a racetrack will end up using two sheets of 3/4" plywood, and that is plenty turdy. We were actually pleasantly surprised by how sturdy ours turned out.
Good luck with your table!