I use a 20cm x 22cm lasagne dish and it just fits!
Put half of the lentil and vegetable sauce in the bottom of the dish.
Cover with a sheet of pasta. You can cut up the sheets and piece them together to fit.
Spread over half the cheese sauce.
Cover with another layer of pasta.
Add the rest of the lentil and vegetable sauce.
Cover with a final layer of pasta.
Spread over the rest of the cheese sauce.
If I have any cherry tomatoes or basil in, I use them to decorate the top but it isn't at all essential.
Sprinkle the grated cheese over the top.