Multisize but the PubMaestro placement has a size

In this case the publisher wants 1 single ad slot that can be filled by any of 2 sizes.

The problem is that in PubMaestro the placement is defined with a size of 768x1024. The correct way is to configure the placement size as "Non-standard/Fluid Size"

Display ad - 768x1024 instead of the desired 300x600 or 300x400