Yeah, this is the one that I have had the hardest time explaining to people. The diameter of your rotisserie cake is limited only by how tall your baking area is. The length of your rotisserie cake is limited only by how long your baking area is. We could theoretically build incredibly long ovens, churning out fifty-foot-long spitcakes, and/or incredibly voluminous ovens, producing cake monsters that are wider than you are tall. Giant cakes.
I never had a really good picture to illustrate this concept, until tonight. Ethan R. found the picture that I've always wanted (but never thought existed). It's part of a story about Lithuanians (my people!) baking a world-record Šakotis. Šakotis is the Lithuanian rotisserie cake! The primary difference between it and baumkuchen is that šakotis batter is dribbled unevenly onto a conical form, and baumkuchen batter is brushed evenly onto a cylindrical form. But I digress.
Look at that. It is the 67.4 kg (148.6 lb), 2.3 m long (7.5 ft), .5 m diameter (1.6 ft) triumph of pastrydom.