If so, 2 sides are 700mm and two sides are 400mm. There is also an MFS400 where two sides are 400mm and two are 200mm.
Not sure of the maximum opening, but you can also buy 200mm, 400mm, 700mm, 1000mm and 2000mm lengths, and can join multiple items lengthwise using joiners so you should be able to get to any size you need.
Take a look at Brice Burrell's guide for some excellent information and further links.
With an MFS700 you can cut an aperture approx. 664 x 600 and also 764 x 600
Join the two 400's end to end and then create a "C" shape with resulting "800" and the two 700's