My Ikea plates with a diameter of approx. 27 cm just fit in. Much more is probably not possible. The available inner height from the turntable is approx. 16 cm and completely unsuitable for jugs. But my plates with a cover really do fit just inside. In the end, the dimensions are really very small.
I think it depends on the bun. The microwave has microwave, grill and the combination thereof. If your roll allows the corresponding preparation or there is a description on it, the microwave won't be picky :) However, it will definitely not be as good as in an oven or microwave with convection function.
Here is the manufacturer's website: https://coldtec.ch/ However, I did not find the microwave there. You can find the data sheet here: https://cdn.competec.ch/documents/5/2/520927/Datenblatt.pdf