For most trail and all-around mountain biking, 180mm rotors hit the sweet spot. They deliver solid stopping power and manage heat well on moderate to steep descents, without adding unnecessary weight to your bike. That's why 180mm is the most common front rotor size you'll see on trail bikes straight from the factory.
If you're a lighter rider or mostly ride cross-country on rolling terrain, 180mm front and 160mm rear is a very common and effective setup. Heavier riders or those tackling long, steep descents may want to step up to 200mm up front for extra bite and heat dissipation.
The good news: 180mm is compatible with most forks and frames without adapters, and upsizing later is one of the cheapest brake upgrades you can make. Just verify your fork and frame can accept larger rotors before making the switch.