Doors are normal size. Taking her in and out, she almost brushes her sides....now
Is there any way you can keep her out 24/7 with shelter and possible blankets? Its more natural for her to be able to move around at will, and you avoid the risk of her bumping her tummy against a wall of something.
In 1999, I was told that one of my mares could not possibly have twins when she became unusually huge at about 6 months gestation. When she bagged up and started dripping milk at 8 months, we checked her for placentitis and immediately put her on Regumate and SMZ. There was no thickening of the placenta, so placentitis was unlikely.
I then took her to the local veterinary college's clinic where they had more powerful ultrasound machines. They discovered twins of equal size, each in its own horn of the uterus. This was a small, short coupled mare who had had post foaling hindleg paralysis after having a large foal lie on nerves. Having twins was a real danger for this mare, as it would be for any mare. But her size and shape made her even more vulnerable.
I was given two options - to abort her under controlled circumstances at the clinic or to take her home, continue meds and wait, hope to get her back to the clinic for foaling and hope for a safe foaling (with the percentages far from good for either foals's survival). I opted to abort for my mare's sake. All went safely, but was sad beyond words.
I learned that year that vets can make mistakes and no matter how adamantly they might say that twins are not possible, they can be wrong. Twins get missed, late ovulations get missed.