Sizes
Application dependent. Mirrored surfaces from 0.5mm to 300mm  
Surface Roughness
Superpolished (0.05nm rms) low scatter or conventionally polished (0.5nm rms)  
Flatness
Up to lambda/40
Materials
Standard optical substrates and metals. Typically fused silica, BK7, CaF2, MgF2, Cu, Al, silicon carbide, sapphire.
Coatings: metal, enhanced metal, dielectric