Various thicknesses, 4mm or 6mm; 

Length and width - no pieces smaller than 20cm x 30cm (maximum 55cm x 60cm)

(i.e. larger than the longest side of an A4 sheet in at least one directions).

...if you have a preferred minimum size do let me know and I'll see if we have what you need.

Basic rectangular sheets some with corner cut off.

10 kg (30-60 sheets) Mixed colours (White, Green, Blue)

Some markings, easily cleaned up.