Tire/wheel size on 2010 silverado

Ok so I know this may be a shitty place to get advice on this since most people probably wont know an exact answer.

I have a 2010 Silverado 1500 and it is sitting stock right now. I will be getting a 2.5 in level kit up front to get rid of that awful rake. I will also be getting new wheels and tires

I was thinking about 18x9 or 20x9, so can anybody tell me which one would be better, I know that 18x9 will have more tire obviously but are there any other benefits? ***also I want to know the biggest tire I can fit on there, I was wanting to get bf goodrich all terrains in 285 which is basically a 33, but I DO NOT want to trim anything. Thank you to anyone who can help out