We used the big toast (American) from Lidl, but the smaller ones from Migros, both work very well.

The size is intended for the American sandwich, which works very well. I honestly haven't tried it with the smaller toasts, but I assume that they are not optimal. Because the edges are not sealed properly. But...I haven't tried it yet.
Extra tip: Don't overfill the toasts... it makes a big mess ;-)