It certainly seems a little tight. The wide end of the inner, at 140cm, is only just wide enough for 2 mats, and the measured 97cm (which roughly agrees with scaling the drawing) at the foot end is narrower than a pair of any rectangular mats I've seen. As the total length of the inner is 215cm, there isn't enough length to simply avoid putting full-length mats into the foot end.
Here's the drawing with a few more dimensions from your measurements and scaling off the picture.
You have a few options.
- If you're using foam mats, you can cut them, for example tapered as you say.
- For backpacking in mild conditions you can cut foam mats short, to run down to your hips. You can also buy them like this, and save a lot of packing bulk.
- Many air-filled mats are themselves tapered, whether self-inflating or blow-up. As an example, here's the one I have. Note that the description says "shape: mummy", which matches the description used for sleeping bags that taper at the feet, and indeed those pictured.
- Thin mats of any type can be overlapped a little, or some may bend. This is useful if you're caught out, but not something I'd plan on except with the very thinnest foam.
- As the sleeping compartment is longer than most people, sleep with your head right against the inner door, and use the narrowest space for stuff. That's worked for me in a rather small tent even with 2 tall people, combined with overlapped or cut-down foam.
Where you'll really struggle is using one of these tents with big thick rectangular air-filled sleeping mats (example very similar to one I've had), which are often long as well. Many of those are too bulky and heavy for anything but car-camping. Unfortunately plenty of camping shops sells lots of tents and sleeping bags this shape, but only a few truly compatible (as sold) sleeping mats. I suspect this is because people often buy a bigger tent than the number of people would suggest (e.g. a "3-person" tent for a couple). Many, but by no means all, tents are rather cramped when fully occupied.
Always test this sort of thing before you leave - you've spotted this, but there may be other things to catch you out.