the army list should have suggested base size. I see the unit listed, but i have no clue how the unit suppose to look like. Probably if you know all warhammer codex book its obvious for you, but not everyone growup studying wargames.
Because OPR is minature agnostic it's up to the player to determine what fits best. This is usually the base the models come with. Because this is AoF:R you'll probably want square but you could do circle.