If the 5x7 paper size is not supported by your printer (that is, it's not
listed as a specific size, and you have to use "Custom"), then your ability
to print to it will be somewhat limited. On my printer, for example, the
paper guides for small stock converge in the center, but all "Custom" sizes
must be aligned on the left. There are two ways to get around this: (1)
ignore the paper guides and put the paper where the printer expects it to be
(which is what I usually do) or (2) set the document up on a standard size
(such as US Letter), adjusting the margins to position the print area to
correspond to where you will feed the paper (in my case, this would be in
the center).