Depends a lot on what you are looking for. RTS games differ quite a lot in terms of what mice is best for them.
Slower paced RTS or turn based RTS do not require as high precision and speed.
Fast RTS games favour small mice since its better to use finger grip or claw grip rather than palm grip.
In theory, if your hand rests over the mice, your arm and wrist controls the movement, so you will have to move more body to play (arm basicly).
This can become an issue since your arm can get tired faster tahn just your wrist and fingers.
To recommend a decent mice, I would first need to know what kind of grip you favour, and what RTS games you play more.