The pan will usually be 8 inches (20 cm) wide by 4 inches (10 cm) tall. The raised “mould” area rises up about 2 inches (5 cm) into the pan, leaving a leaving a 2 inch (5 cm) deep moat all around it.
You turn the cake out of the pan in such a way that the bottom of the cake becomes the top. This reveals a large cavity in the middle of the cake about 2 inches (5 cm) deep where the mould was. This area is designed to hold a lot of filling, which can be fruit, American-style puddings, lemon curd, whipped cream, ice cream or custards.
The pan will hold about 3 cups (750ml) of batter; when baked, the cavity will hold about 2 cups (500 ml) of filling, giving you a 3 to 2 ratio to work with.
Mary Ann Pans are very similar to a German “Obsttortenform” (aka Flan Pan), except Obsttortenforms tend to be shallower, and whereas Obsttortenform edges are always fluted, Mary Ann Pans may have fluted or flat edges.
Tiara Cake Pans are also similar, but have a shallower depression, are wider, and don’t hold as much filling.
You can also buy miniature Mary Ann Pans, which are 3 1/2 inches (9 cm) wide.
If you are making a Mary Ann from a cake mix, divide the mix package contents in half and use only half (and add only half of any ingredients the directions call for.) The cake will probably be done 5 to 10 minutes sooner than the directions for the cake mix indicate.
Mary Ann Pans seem to have first been advertised in 1921 in America.