Does anyone know what size of metal mesh shoudl I use to block out RF frequency? If not, what is the shortest wavelength of RF that affect amplifier operation?

(and from there I can derive the wirescreen: The rule of thumb when building a Faraday cage is that it should contain no gaps larger than λ/10, where λ is the shortest wavelength that is to be blocked.)
Ibanez SA-120 (ed.2006)
BluesJr 1996-B + cathode follower + texas Heat
Crate CPB150
Homemade 4 x 10 cab Bass closeback
Metal Muff
Good question. That would depend on the circuitry, chassis, grounding, location (in the basement, upstairs, or next to a transmitter or microwave.) I would go with the smallest you're comfortable with.
Generaly radio waves are around a meter in length, and microwaves are around 1cm. The metal grid in the doors of microwave ovens are small enough to stop those getting through, so you wouldnt need to go any smaller than that.