Light Aircraft Association

What would be a good size for an aerial ground plan in a plastic plane ?

As large as you can fit.
But I would suggest a minimum of 6 x 6".

Is this to fit an antenna to an already constructed aircraft?

Tony

Yes, But good a access to rear fuselage.

Radius of the ground plane should be at least 1/4 wave length, and flat. So if it is in the curved inner skin of the turtle deck, you will need to widen it, about 0.6M radius or as large as practical.
Simply put the radius of the ground-plane must be greater than the height of the antenna. The ground plane should be horizontal in flight and the antenna perpendicular to it.

Raked antennas may look cool, but they degrade radio performance. However a small amount of rake forward if top mounted or back on the underside would be acceptable.

Oh, and get the VSWR checked. I have seen off the shelf antennas fitted with no tuning. They were supplied for the Nav band which is lower in frequency and needed to be trimmed.

Deleted

I was going to make the same suggestion, but wanted to keep my post to what was asked by the OP and not confuse things. You do have to be mindful of control runs.