Question about risk sizing on Polymarket

I've traded forex a bit, but I'm confused about risk sizing on Polymarket. We bet on what will happen, and there's already a % indicating the probability. But sometimes the market swings a lot, and it seems some people focus heavily on counter-trading or trend-following. I'd like to know how you guys determine the risk proportion for each trade? Do you have any principles you use, or do you just focus on probability?