Thanks for the comments.
Checking my gear, it seems that I need to add about 100 mm at both the head and foot ends of a body to allow for the thickness of the bag or quilt. Beyond that the fabric touches the end walls of the groundsheet (NOT the tent), but that does not collect condensation in my experience. So a 2,200 mm long groundsheet caters for a person up to 2,000 m (6' 7") tall - which is going to cover an awful lot of the population.
Looking at many other tents on the market it seems that there aren't many with a groundsheet longer than 2,200 mm, which is encouraging. It should be noted that the longer you make the groundsheet, the bigger the footprint or tent site required. That can be a problem in some places. However, not final.
What is more important is to ensure the tent is wide enough for two people - plus some gear down the outside edges. Having some gear space at the sides is very important in my experience. The gear insulates the side of your quilt and stops you rolling off the mat/pad as well. My previous suggestion of a width of 1100 mm seems to be definitely inadequate. (Mistake, actually.)
I don't think there is any chance of a 60" width for the groundsheet on the summer tent - this is meant to be UL after all, but it certainly should be a bit wider than the 1100 mm. Of course, as the tent gets wider it gets heavier. But 1250 mm width might be more appropriate? That gives ~100 mm on the outside of a mat, each side, for gear.
Staying under 1.2 kg for tent+poles will mean we have to look very carefully at the fabrics used. That will take some time.
By the way - while my shoulders are wide enough, my feet aren't. So in practice there is an acre of space down at the foot end of the tent for any amount of gear.