Peter, I looked that the 8 inch option and worked out that, on my Plus4, the lower part of the 8 inch wiper blade would foul the windscreen rubber/frame at the bulkhead. So did not go ahead with that option.
I think it all depends on the position of the arm attachment on the blade: if proportioned as on the OEM, with a longer part above the attachment than below, it will fit.
Based on the dirty pattern on my screen I have nearly 5/8" clearance "free". So a blade that is overall 1/2" longer should be fine.
The only certain way to know is to try....
