I recently asked on the Facebook, GF9 Tanks: Modern fan page, if anyone plays of a larger than 3' x 3' table in a variant.

Scott Savory used a 4' x 6' tabletop, at a 150 points per side...

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...the building are from Battlefront's "Battlefield in a Box"

Other sizes mentioner were: 4' x 6'; 4' x 8'; and 3' x 6'.

4' x 4' was a very popular size, with 200 point armies, such as Ian Gardener's....

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I'll be looking to try these sizes out!