Why size a position backwards?
Most beginners place an order the other way round, thinking "I want to buy 1,000 worth", and how much they lose comes down to luck. Risk management flips the order: decide first how much this trade can lose at most (say 1% of the account), then look at how far the stop sits, and work back to the quantity with "acceptable loss ÷ loss per unit". However the market moves, the loss on the trade stays within your plan.
The formula is simple: acceptable loss = capital × risk percentage; loss per unit = |entry price − stop price|; suggested quantity = acceptable loss ÷ loss per unit. If the position comes out much smaller than you pictured, that is not the tool's fault; the stop is too far away or the risk budget is too small, and both are worth rethinking.
The method works for both spot and futures. Futures users also need to keep an eye on the liquidation price, so pair this with the Liquidation Calculator; if the concepts are new, read Futures & Risks first. The calculation excludes fees and slippage, so leave a little margin in practice.