Some small dice are needed to keep track of the strenght of the units. Since you place them next to your unit on the gameboard, they should be small enough so you can put them on the same hex and big enough so you can handle them easily. 5mm is probably too tiny to pick up between your fingers and 12mm is a bit large compared to the size of the units. 7mm is a nice size and is easily available.
Instead of dice you can of course use chits or counters, but dice are quite a bit easier to pick up and change number while playing the game.