Hi,
I´ve played in 20 mm too and we used an 1,2 x 1,8 m table. We did not change any movement or firing ranges. The game was played with about 60 miniatures per side.
For small unit games I use a 1,2 x 1,2 m table.
Our 40 mm Tactica participation game will also be played on a 1,2 x 1,2 m table but with only about 16 Miniatures per side. No change of movement rates in this scale too but we said that all rifle and musket fire is in range all over the table when you have LOS to the target. Rifle shoots without penalty. Musket - 1 at ranges over 24 inches !
Cheers
Björn