IMO, the most effective glide slope control is flaps in combination with traditional (pop-up fence) spoilers which destroy lift more effectively than reflexed ailerons in crow, while also preserving aileron authority at low speed. The drawback of spoilers is that they require at least one more servo, and can sometimes (read "usually") be a hassle to install.

It's not realistic to predict how much roll control you'll sacrifice without knowing several other things: the plane's inertia about the roll-axis, aspect ratio, flying speed, dihedral, airfoil, etc. ==> Yuk. Avoid the headache and just look at several models similar to yours and go from there. I think you'll be safe with ailerons as short as 1/3 of span, flaps the remainder (2/3) of span and flap chord = 15% - 20% of wing chord. It would also be easy to just temporarily tape-hinge flap/aileron stock of different lengths and see the real-world effects.

Narrow long control surfaces are generally better than wide short ones, since a wide surface alters lift coefficient only slightly more than a narrow one, but the load on the servo will increase by the square of the width. Flaps are a high-load application, so servo torque needs to be considered.

If you're intent on using flaps, it would be wise to invest in a programmable transmitter if you don't have one. Deploying flaps almost always causes a pitch-up situation requiring down trim to maintain level flight - a pain to deal with manually.

When flying, one thing to be careful about is rapidly bringing flaps up, especially during landing approaches - A rapid reduction of Cl when the plane is already flying slowly can cause a nasty stall; the last thing you want at low altitude Similarly, large flap deflection can cause a plane to fly so slowly that it becomes prone to tip stalling or sluggish control response - Find out how your plane responds to these things when it's up high!