When using panels, usually people use 4 foot by 8 foot panels. (8 feet tall is perfect for height). Thus 8 X 8 or 8 X 12 is common in size.
If you do it your way, with tarps, you are not limited to 4X8 panels. However most things you might buy as beams (wood at least) come in 8 feet long. So 8 feet tall makes sense.
Also, a simple cube 8 X 8 X 8, is easiest based on the existing lengths, minimizing wood cutting.
A table is usually 36" (3 feet) wide, so a 4 foot wide succah is probably too narrow.
I would rather consider the space you have available, and build to that. Remember you only need three walls, not 4 (and technically, one of those 3 can be very short/small).
Simplest approach is build a basic frame, I like bolts in 2X4s since it is easy to reuse next year. What I would recommend at each of corner to put in two bolts, otherwise, the square becomes a rhombus very quickly. Other approach is to use a diagonal brace in the each corner.