Hi,
A standard 8.4 volt Robbe Navy compact number 4105 at 9.6 v with a 45 mm racing prop draws 5.5 amps loaded, obviously this will increase if you stall the prop, I believe the standard Robbe navy compact has the 3:1 reduction fitted and is quite a powerfull unit ( the navy compact is the one with the shaft attached to the gear box you just bolt the motor onto it )
The easieast way to determine the power drain is to place an ammeter in line with the battery to one of the motor terminals with it set on the amps reading, you will then know the amperage draw of the motor , try and increase the drag on the motor until you stall it if you can and this will give you the max amps at stall, a 10 amp controller will be fine for most applications.
Hope this is of use :)