What do you need to ensure that sketches are of a suitable size and use appropriate drawing conventions?

What they are asking for here is some recognition of parameters for the artwork. While the lack of data here precludes a specific answer, see that an 8 1/2" x 11" sketch of a mural that might cover a whole wall might not be 'suitable;' or alternately, if 'suitable size' has been defined, that sketches do not exceed this parameter.

'Appropriate drawing conventions' are defined according to the specific type of sketch that is being done; an architectural sketch would necessarily use different conventional parameters than a portrait, for instance.