hjulenissen and torger,
The big problem is not just making big sensors, but making them with photosites small enough to be of interest. There are many very big sensors available for uses like X-rays and astronomy, but they all have big pixel pitches like 15 microns and up ... often way up, like 160 microns. In 6x7 format, 15 microns gives about 4600x4000, unlikely to be competitive in IQ with what modern near 645 format sensors are giving.
The reason is that to get pixels small enough to even match film resolution requires using modern semi-conductor fabrication equipment all of which has standardized on a maximum field size of 26x33mm. Using these to make a sensor larger than about 24x31mm requires a complicated, low yield, expensive fabrication process of "on-wafer stitching". (as you probably both know, this is _not_ making smaller sensors and then joining them afterwards.)
On the other hand, for lower resolution scientific and manufacturing needs, there are cheaper options like an old, low resolution stepper by Canon with large 50x50mm field size, or butting together several sensors and ignoring the join lines (acceptable for X-rays) or using steppers designed for LCD fabrication which have very large formats but very low resolution. But the ability to make those large, very low resolution devices is not the slightest evidence that there will ever be higher resolution semi-conductor fabrication equipment capable of make a high quality MF sensor without on-chip stitching.