i'm a big tin lizzie advocate. 18" and i believe the grace will hold the weight.
BUT, how big a machine will actually be able to work on the work area of your frame. the frame has to have been built for the throat size. i can't use my frame, that now holds an 18" machine. for a 24" machine and still have space to work. the front end of the base of the machine would bump into the front roller. so there wouldn't be a lot of back/forth movement. try some machines, see what you like, go to their blogsites and see if anyone is selling. lurk for a while and see what they like and what they don't.. you get to learn a lot that sellers don't tell you.