Note - slightly larger pieces of beef are fine, you just may need slightly more fillings. If your pieces of beef are particularly big, you can add more filling to each and serve one per person rather than two, as here, since they are smaller. For all, they should be thin and it's best to get your butcher to cut the slices thinly for you rather than try to do at home.
If you prefer, you can cook the rouladen in the sauce on the stove over a low heat - overall timing and turning now and then remains the same. Just if you do this, you'll need to take extra care that they cook evenly and don't brown too much on one side.