Radiator opening size

Way too many variables to even guess.
It will depend on the location of your radiator.
Is the engine a Pusher or Tractor?
Does it share the air with the Cylinders?
Is it before or after the Cylinders?
Is the Air entering the intake square to the air stream or at an angle?
Is the intake Before or After the Prop?

Yes, more questions than answers. To answer a few, the radiator is inside the engine cowl at 64 degrees to the airflow. I am planning on using a honeycomb  airstraightener at the face of the radiator. It is a tractor configuration, airflow will be after the prop fed air directly though appropriate opening in the lower cowl ( which I am a fan of because it will be more frontal to the relative airflow during a climb). The radiator intake will be seperate from the rest of the cowl air but will exit through the same openings as the cylinders etc. Projected climb speed should be around 70 knots

You may get several different opinions and ideas here, but there are different ways to accomplish your task.

Sounds like you're on the right path. You'll see many different configurations from different companies and experimental owners. The more forced air through the gradient the better during hotter months. 

I'd make the opening the size of the radiator. You don't need to go out passed the solid edges of the radiator, but just the pass through area. Many will put baffling around the edges.  GENERALLY speaking the more unrestricted air through the radiator the better the heat transfer.

Flight Design has a setup that describes about what you're doing. The small round hole behind the prop is for the engine plenum and the elongated opening is the radiator with the oil cooler right behind it.

Thanks Bill and Roger.

I found some specs I could use as a guide. Rotax states the radiator need 0.75 cu.m/sec. of airflow. If I make an opening of 5" x 9" (Which will be tight  for space) that should give the required airflow at 54 kts or a 4" x 8" opening would require 76 kts to give enough airflow. The prop blast is more interesting, at 5200 rpm (81" pitch), the airflow is almost double what is required with a 4" x 8" opening. I wonder if this would work out in the real world though, frictional loss, location in the airflow etc would be a factor. 

Go as big as you can!
A lot of designers get into trouble by not considering waiting in line for departure on a hot summer day.
Heat loads in-flight might be the easy part.
Consider how you will handle taxiing 3/4 of a mile at idle with a tailwind.
Design for the worst case!