I can't say without knowing much more about your engine build, fueling system, compression, cam timing, fuel grade, ignition control, etc. Turbo sizing is a match between all these factors and your goals. A dyno sheet and full description would be best. A full listing of components to determine general engine traits is minimal. If you have not built yet, it is always suggested you plan the build starting with specific goals and based mostly on the fuel you plan to use.... I am only looking to run 7 to 10 psi since this will mostly be street driven what would you recommend.
Charge cooling is recommended above 5-6 psig as the general break-point. Meth injection? I assume you mean water injection and not using methanol for your primary fuel. Water injection is a different animal that retards and modifies charge burn for greater power by allowing either more proper ignition timing, or more boost, or both. Many add alcohol (usually ethanol) to the water and don't really know why, but I sure hear a lot of fascinating theories. LOL The intercooling/WI effects seem similar and overlap. Charge cooling is first choice for increased charge density, and adding WI due to poor fuel, ignition control, or other factors can also help. Both allow more power with detonation suppression, but for different reasons.Also if I use meth inject with this lower boost would you recommend a intercooler or not?
Intercoolers are a one-time investment, nearly maintenance-free, and very reliable. WI is more complicated, has ongoing costs and maintenance, and has a potential for greater failure rates with costly consequences. My current build uses both, but I understand the failure risks, so I have my EFI system detect if there is low water or a WI failure and will cut boost to protect the engine.