Reply to comment:

Emile Aben
We haven't tested configurations with MTUs over 1500, and currently don't see probes with MTUs over 1500. Also currently the maximum allowed packet size is 2048 (enforced by the RIPE Atlas infrastructure). If there is interest from the community to support probes with higher MTUs and tests with larger packet sizes, we can put that on our roadmap for RIPE Atlas.