Ступня - is the surfase of you foot facing the ground when you stand/walk. Нога - the whole leg from ступни up to where your legs begin) Also нога may meen 'leg up to the knee' or 'up to the ancle' (стопа) - sinse we have no separate word for 'foot'. Size of boots (размер ботинок) in Russian corresponds to 'размер ноги' (feet size) or rarely 'стопы'.

Нога is either foot or leg Ступня is the sole of the foot