The Define R6 can support motherboards up to 285mm (11.2") in width which will support the majority of E-ATX boards that are currently on the market.
*Please note that any boards that are wider than 250mm will block portions of the cable routing grommets on the right, but you can still pass cables through the HDD mounting area when in Standard Layout.
So SSI EEB boards that tend to be 330 (13") wide will not fit, but SSI CEB board that tend to be 267mm (10.5") wide could fit.
However, some SSI CEB boards have a different standoff layout than ATX form factor board do. So keep in mind that some standoff holes may not be used.
Pictures for reference:
Motherboard: Gigabyte Z370 Aorus Gaming 7, 244mm wide (The red box indicates the space that would be taken up by a 285mm wide board)