They only do standard sizes but I can put a border on the image in white so that when you cut that off you get the size image you want. Standard size would be 48" x 36" without a border and I could put a 51"x37" image inside a 60" x 48" border (next size up).
I'm not going to be able to do anything until the end of the month after my vacation so there's plenty of time to think about it.