My first plane had the smallest engine recomended. It flew, and I learned to fly it and learned basic aerobatics. We upgraded from a 40 to a .61 boy what a change that was.
The models will normally fly just fine with the recomended engines. You will not be able to get the wild aerobatics, or 3d flying with them. But if the kit calls out a .40 as long as you dont build heavy, a .40 should fly it.
About everyone today goes by the motto, if it calls for a 40, it gets a 60,or if its on sale it'll get the 90. People like to fly around at 1/3 throtle and be at the aircrafts N.E. speed.