Hm, theoretically there is no limit (minimum or maximum) to the size of a battle grid - in practice you'll need at least enough cells to place all combatants that are participating in the battle.
E.g. if your grid only has 1 cell, you could only place one combatant on it, so there isn't much of a battle ...
Edit: Yeah, seems like anything below a 4x4 grid is causing issues - will be fixed in the next update.