4 x 4 is plenty for a 2-3 hour evening game, bigger than that I dont see getting a result. I have played on a 6 x 6 with 3.5 hours for the game and 2 players a side, we need at least another hour or so to get any sort of conclusion.
If you think that movement is typically two D6 and you cant move if your firing or re-loading its going to take 3 or 4 moves to get into action even on a 4 x 4. At long range after the first volley firing is typically very ineffective so you need to get reasonably close. 4 x 4 or 4 x 6 are ideal table sizes for me.