Our Winds Discipline Specialist has provided the following response to this question:

Regardless of a candidate's age, each examination performance is evaluated based on a full range of considerations: quality and control of tone, command and evenness of technique, intonation and rhythmic precision, dynamic range and contrasts, and musicality. However, there is no formal breakdown of how the marks are allotted amongst these various considerations. The higher the level, the more the basics of sound production will be expected to show refinement. A Level 5 examination is an intermediate grade, and so there would be some expectation of tonal control, but not as much as at a higher grade level.

A candidate's size on a brass instrument does not necessarily have to affect their sound quality, if they have made progress utilizing a full and free air stream.