My rabbit is 4kg and he has a 48cm long carrier for himself (important theyre not too big so they dont go flying against the side in the car) but will be getting a 58cm one when he has a friend, although his friend will be a small bunny around 2.5kg or less.
Hope that makes sense.
If you can carry it, its best to have them in carriers together I think, keeps them calm if they have their friend there. Or if its too heavy you can get ones with wheels.