As a general rule, gas dominates in planes with 80 inch wingspans or more. A gas powered plane with a 72 inch wingspan is a small gas plane. Glow is used in smaller planes. The trainers you will learn to fly with will probably be powered by a .40 or .46 cubic inch glow engine. Cox once made a tiny little .010 cubic inch engine that's now a collector's item.

Below 20cc (1.2 cubic inches), glow ignition dominates. There are smaller gas engines but they aren't common.
Between 20 and 30cc, it can be either one.
Above 30cc, gas dominates. There are glow engines bigger than 30cc but they aren't common.