What’s wrong with 75mm - most of my DIY Kosmo modules are 75mm. My 3 Filters, both of my 3340 VCO’s and my midibox sequencer interface are all 75mm. The “spec” just says “multiples of 2.5cm” as far as I see. And 75mm is just 3 2.5cm units. Sam may not have produced any 75mm modules but I think it’s about the perfect size for most things. 50mm is a bit tight for a lot of things…and 100mm is too tempting to cram too much in there 
I say we need more 75mm modules 
That said…I probably would have gone 100mm on that particular one because 4 jacks across in 75mm IS a bit cramped for my tastes But I also tend to like multiples of 3’s and most of my modules are therefore setup with 3 columns for jacks/knobs/switches which I find very pleasing at 75mm.