What is the first number you check when sizing a trade, and why that one?

Before you think about entries, it helps to know what a single wrong idea would cost you in dollars. For many traders that means fixing a maximum loss per idea first, then letting the distance to invalidation determine position size, rather than the other way round. I find the order matters. If you start from a size you like and then place the stop wherever it fits, the stop tends to drift to suit the position instead of the chart. Working backwards from the loss you can accept keeps the invalidation point honest. I am curious how others approach it. When you size a trade, which number comes first for you: the dollar amount you can afford to lose, the percentage of equity, or the distance to the level that proves you wrong?