How do you define 'reasonable' risk on any given trade?

Been trying to get my head around risk sizing lately, beyond the simple 'X% of capital per trade' rule. I understand that. But what does that X% really mean in terms of a dollar amount you're comfortable losing on a single position? For example, when looking at a setup, say $EURUSD, and I see a potential invalidation level, how do you decide if the R:R is attractive enough to warrant risking a larger chunk, or if it's better to just pass and wait for something cleaner, even if the math works out to a positive expectancy? I'm finding it hard to internalize what 'reasonable' means for me personally.