In my opinion you should go with the longest jig you can build. If you went with the shorter one, sure enough you will be saying " damn I wish I built this thing a little longer". As far as width, I built mine at 3.5 feet. I notice you have what looks like movable cross members, I used this exact concept, thats why I made the jig a little narrower, the cross members can make up any width issues. I set my height at 24" and it is perfect, (I'm over 6' tall however). Except for the main rails, I used 3x4 1/4 wall tube, you jig has the same basic design as the one I built and I have had no real problems with the layout.
Hope this helps