Lift something that weighs 10 lb - e.g. a gallon of water weighs about 8.3 lb plus container. Then imagine exerting that force at the end of a 1 foot wrench handle, that is 10 ft-lb. For a six inch handle, 20 lb would be the same torque (1/2X the length needs 2X the force).

Or, for a typical adult male who isn't unusually strong or weak, if the threaded part of the bolt is the diameter of a standard cigarette or pencil, hold the wrench handle so close to the bolt or nut that your thumb can wrap around the bolt or nut. If you tighten using your wrist strength only - so that your elbow doesn't move at all, no shoulder or whole-body involved - you'll be hard-pressed to over-tighten the bolt. If the thread is the diameter of a Sharpie, move your hand further out the handle until your thumb can just barely touch the bolt or nut, and then same idea with the wrist strength. If the thread is smaller than a cigarette, hold the wrench close to the bolt or nut (usually the handle will be short anyway) so that your thumb is on the bolt or nut and just two fingertips are pressing on the wrench handle, then tighten with only wrist and those two fingers.

If either the bolt or nut or whatever is being screwed into is aluminum, then be particularly wary of stripping the threads. The wheel nuts and axle on your Walmart bike are steel, and pretty large diameter, so will be hard to strip those threads.

If you are really concerned, spend $4 for some threadlocking compound (autoparts store, hardware store) and use that when assembling the bike.