First Build - Yet Another Alu Profile Router - 900 x 800

[JAZZCNC]

Like this?

My gantry riser plates are currently 210mm width so this arrangement would fit. My concern is that the CoG of the router would then be well in front of the forward Y-axis carriage. Is this what you mean when you said about wider bearing/gantry plates? ie. I'll need to widen them to bring the CoG back within the two Y-axis carriages? Which would then mean a reduced cutting area?

Yes but not so much because of COG but more the loss of travel. COG isn't a big deal as your probably thinking and you are not exactly hanging it out into outer space.
That said I'd also consider looking at 160 flat with 80 on top and rails top n bottom. You'll lose some height between bearings but gain in stiffness.
End of the day it's down to what you need in terms of clearance between gantry and bed, along with travel required. Also what you are wanting the machine to do.?

My suggestions are based on what profile you have, however, if I was building it and wanted a very strong gantry at this size I would use 80x120.

Option E will work fine as well if you cannot afford the length. I wouldn't stress over any of them because each will work fine for a router.