How the grid lay out works is that
you start with a div row,
and you can split that into twelve columns. You can split any div row into 12 columns.

http://foundation.zurb.com/docs/components/grid.html

To center this form, I would probably create:
1. div row,
2. div medium-x-centered columns 
3. div row,
4. split this row into 2 6column-sized groups. One for the left, and one for the right of the form.

small/medium/large is solely about responsiveness. If you put small, it will maintain the grid layout on small,med,large screens. If you put medium, it will maintain it only for med/large, and then split the rows for small screens. You can see this happening on the 
http://foundation.zurb.com/docs/components/grid.html really easily when you resize the columns.