These cups are a large 'hat' shape, and you cut them down to the size and shape you need.  The cups have a CF and bottom centre indicator and come in a pair.

They have a radius approximately 25cm(10") from the outside to the centre (diameter 50cm (20")

You would cut the cup from the centre, so smaller sizes may find a lot of 'waste' around the cup, but this can be used flat to cut other cup pieces (used as flat foam), as its non-stretch, so no grain to consider.

They are a designer professional option, so if you find them too tricky, you can always cut the cup in half and just use the foam! (as per the picture of one cup cut in half below)