Question on position sizing for multi-asset strategies

I'm still wrapping my head around effective position sizing when dealing with a portfolio that mixes asset classes, say, some long-term equity positions, a few shorter-term forex trades like $EURUSD, and perhaps a small allocation to crypto such as $BTC. My current approach feels a bit ad-hoc, mostly based on a percentage of capital per trade, but it doesn't really account for the varying volatility and correlation between these. How do others here approach a more systematic, risk-adjusted position sizing across such diverse instruments?