I'm not certain what do you mean by size. The amount of units on the screen or the unit variety?
Size itself doesn't attribute to a game being harder or not much. For example compared to Starcraft 2, Zero-K is much easier to play and that game has way less units under your command and having less unit options due to having factions.
If you want an "easy" RTS with "few" units I recommend Leauge of Legends :)
(not DoTA2 though because it has some heroes that actually require quite a lot of micromanagment)