How big do they need to be? Nissan has many options, but most of them are 5 lug.
Will it be a legal problem drilling 3 extra holes in the rotor hat so you can mount a 5 lug rotor on your 4 lug hubs?
In the US, the P10 comes with 257x22mm and the P11 comes with 280x22mm, but the calipers themselves are the same. If you are looking for a visibly larger caliper there are plenty of 4 piston sumitomo and brembos to choose from. Wheel clearance might become an issue as the 4 piston brakes are bulkier on the outside.
Largest 4 lug I know of would be the Brembo option from a 2004+ Sentra Spec-V. They bolt onto the P10 as they are from a FWD car. Rotors are 305x22mm Calipers have 4 pistons, 2 each of 38mm and 44mm.
Other options would be 280mm brakes from a 300ZX and some versions of the skyline. Although not any larger in diameter than a P11, the rotors are thicker and come in either 26 or 30mm thick. It doesn't add leverage because the diameter is the same, but the additional mass helps absorb the heat from repeated stops. They use 4 piston calipers made by sumitomo, but require new brake lines because they use an inverted flare instead of a typical banjo bolt.
Next size bigger is the 296mm brakes on other models of the skyline. They use 4 piston sumitomo calipers that look the same as the 280mm but they are just bigger. I think there is also another version that is made for 310mm brakes.
In 5 lug brembos from skylines and 350z track models are 324x30mm.
Now if you don't need a 4 piston caliper, you could use a 2 piston floating caliper where both pistons are in the inside so it provides more spoke clearance. The come on some of the larger and newer sedans like the Q45, G35. Depending which model you get it from they range from 280mm to 320mm
One thing to keep in mind is if you use a caliper from any of the rear wheel drive cars or awd Skylines on a P10, you will need to shave the rotor down about 4mm in diameter because our mounting points are a little bit inward. If you use brakes from a front wheel drive car like a maxima, then they should work just fine without needing to cut the rotors to size.