Hey my FD is running on HKS Hipermax 3 coilovers, but in need of new tires soon, and thinking about going the beefiest tire size but not sure what tire size I can run.

Currently running BBS RG-R: 18x8 +40 F and 18x9 +42 R. Ideally would want a good staggered setup. What's a good size? Would 235/40 18 F and then 265/35 18 R be the best most beefiest size I can get on safely without any negative under/over steer and whatnot? Any thoughts?

 

Tire Rack lists the specs for the tires they sell. That includes: section and tread width, diameter, rim width and suggested rim width range.
They also explain wheel offset.
Using all of this you should be able to get safely close to what you want.

I run Enkie PF01 wheels 18X9 +42 front, 18X9.5 +45 rear with 155/35-18 and 285/30-18 tires. But my springs are also 10Kg/8Kg so suspension travel is less.

 

Seems like reasonable sizes. Check the overall diameter. I think TireRack lists that in the specs section of the tire you select.