how much power for servos ?
 

I don't like to use standard servos in a acrobatic plane but thats just my opinion. I would use something with a least 50 to 60 in oz of torgue. But on the lighter side I have friends at the field that fly standard servos that never have a problem with them. If your still unsure you can always read the manual and install what the manufacturer suggests.

how much power for servos ?
 

You should use something better than the "standard" (appx 45 in-oz, nylon bearing plastic gear) servos.

The ailerons, using one servo each... you should be OK with about 60 in-oz per servo.
elevator, if single servo, you need 75 in-oz minimum...
The rudder will need either a gang of two 54 in-oz or one 90 in-oz.
The throttle... you could probably use a 27 in-oz mini-servo if you wqant. Not much force on that on. ;)

Your plane will put more force on its control surfaces than the Dr1 I'm flying using an O.S. .91 FS. I had to switch to ball bearing servos all around, and the lowest power servo on a flying surface is a 60 in-oz. The Dr1's got ballanced surfaces all around and the rudder has stripped out one 75 in-oz servo, when I had the throttle mechanically limited to 75%... The Cap is faster... more maneuverable, and has bigger control surfaces. Get better servos.