it matters not you can use a 12 Inch Actuator arm on a 1.2m dish
preferably 24 inch Heavy Arm is much more suitable however ,
on my 2 meter dish i have 36 inch heavy Superjack is not for looks or how fast it'll drive either , its for wind balance
and wind force against the dish as you well know wind will always be a factor
but ground mounted dishes majority of the time will get away with a lighter smaller Arm as they are not so exposed .
Your dish being so high and no proper polar mount will be over exposed but at least summer time is on the way ....