I just picked up a GP Flatout Cub and while I like it just fine, I wish to replace the motor with a cd rom type brushless. What size do I need? Thanks for any help given.

What does it have now, a Speed 400 brushed motor, direct drive to the prop?

Geared 280. I got an email to try the Tower Pro CD rom motors and a 500 to 650 lipo. Made an order from United Hobbies and it should work out OK!
The motor stock will fry eggs when it lands.

Sounds like you already got what you need. Get 'er done!