For me I would probably go with a 4x8 or a 6x12 table, flat surface. For the terrain I would use a combination of WWG Cardstock Models, O Scale Model Railroad Buildings/scenery, and scratch built. Since I play All Things Zombie by Two Hour Wargames, I would primarily be playing City/Suburban setting so the WWG Urban Mayhem or the Mayhem TLX ground tiles would be used. If I did a rural setting them I would probably use the WWG Hinterland TLX set with the WWG, Model railroad and Scratch built buildings for the farm houses and or the small town street (the road would be the Mayhem street tiles possibly kitbashed where needed).
With a 8x4 or a 12x6 table I could do a large city layout or large rural layout if I wanted or even only use half the table for a smaller layout while the unused half is where I have room to roll the dice, keep extra minis, etc... Or I could easily take the 12x6 and use 3 or 4 feet on one end (given me an area of 6x3 or 6x4) as a work space for painting, building etc... while the rest (an area of 8x6 or 9x6) of the table can be used as the actual playing surface.
As it is I have been thinking of getting a regulation sized ping pong (table tennis) table for use as a gaming table. Which I believe is a 9x5 area.