Minimum 2300mm inside length ((maybe not over the full width, but for the majority of the width)! Someone said most tents are shorter than that, and I agree, most are to short!
My fall/winter bag is 2400 long with the hood flat, maybe 2300 with the hood done up. I am 195 cm / 6'5" tall, and there are plenty of taller people around.
The last thing I want in a single wall tent or any winter tent, is touching the walls with my hood or foot of bag, getting wet from condensation and reducing warmth from compressing the down.
Could you add a length-wise diagram same as you did for cross-section?
Width 1250 mm. At least in the shoulder-knee region. Sure, regular pads are only 20"/ 51cm wide, but most people hang out over the sides somewhere or another. You said there are no lightweight wide pads? In the winter I use a long and wide closed cell foam pad, at 25". And then you need to add the sleeping bag loft to that, easily 100mm on each side. I know you have tried it out, but some of us are bigger than you, or don't like to snuggle quite so close to our tent mates.
+2 on setting up your tent, getting two guys over 6'2/185cm and their three-season bags and start taking pictures, even better, let them sleep in it.