I have the same easel, but mine is branded Kindermann. Yes the easel support 11x14 papers, but there are no stoppers for that size so you have to use the ones for 12x16 or 9.5x12. The reason for the spring loaded stoppers is that you can make different enlargements without the need to move the easel under the enlarger as the different paper sizes are always centered if you use the correct stoppers for the given paper size.
I use a Dunco enlarging easel, the large 20"x28" model. While the unit works, the build quality is not really up to the asking price and I would not rely on it for repeatable precise 90º borders.