Rotors come in 140mm, 160mm 180mm and 203mm sizes, and depending on the category of bike you’re riding, it will be set up for a different-sized discs: the front rotor will usually be a size up from the rear.
XC bikes will often have a 160mm rotor in the front; trail and enduro bikes will jump up to 180mm; and downhill bikes or e-MTBs will have a 200mm+ dinner plate.
The size of your new rotor will have a drastic impact on braking power. If you’re looking for a bit of added power, consider going a size up; however, you’ll likely need a mount-to-caliper-adapter to make up the space.
On the other hand, larger rotors are heavier, adapters create another piece to come out of alignment and they are more exposed to trail hazards. Some fork manufacturers also advise against using rotors larger than 180mm on their short-travel mountain bike forks.