You want the mesh basically as tight as you can set it, without it feeling too tight when you push your car. Believe me, you will feel it if it's too tight. There is a way to check it using a strip of paper. You pass a strip of paper between the spur and pinion gear, and the paper should come out with the strip crinkled from the teeth, but not torn. If it doesn't have very clearly defined crinkles, it's too loose. If the teeth puncture through the paper, the mesh is too tight. You adjust the mesh by sliding the engine from side to side, once you loosen the 4 screws on the bottom of the chassis that hold the motor mounts.

The way I check my mesh is I set it as tight as I can without the rotation feeling tight or binding. I then gently turn the spur gear with my finger, and watch to make sure there is no excessive wiggle or delay of the pinion gear turning when I move that spur gear.