Size = (account × risk%) ÷ (stop distance × value per price unit). For a standard forex lot that value is 100,000 (so 0.0010 = $100); for shares it is 1; for futures it is the contract's value per point.
About this calculator. The result is an educational estimate. Contract sizes, pip values, spreads, commissions and overnight financing differ by broker and by instrument, and fills can differ from the prices you model. Check your own account's specification before trading.