What takes new traders out of the market is rarely picking the wrong share — it is entering with a size their account cannot absorb.
The practical rule: first decide the share of your capital you are willing to lose on a single trade, then size the position so that the loss does not exceed it if the price reaches your stop.
That means the distance between your entry and your stop determines the size — not the other way round. The wider the distance, the smaller the appropriate size.
In margin-traded instruments, remember that leverage magnifies the loss in the same proportion as the gain, so calculate on the amount at risk rather than on the margin held.