I googled "proxxon mini belt sander", poked around a little, saw an image that was similar to your machine so I clicked it, took me to a review of it from 2012, in the review they detail the belt size, it is metric.
Give it a try searching for it, the info is certainly out there...
Thanks for the links, I just measured the belt and unfortunately it's 30mm x 300mm, so those belts won't fit, I'll keep looking though. I also got a response back from Proxxon And they said the sander is not a Proxxon product, and to contact MicroMark for assistance. I guess that would explain why it says made in Japan on the sticker above the serial number sticker, now I just have to wait for MicroMark.
MicroMark contacted me and said the sander was discontinued 8 years ago but they did send me a copy of the manual. As for the belts I found this place https://bndabrasives.co.uk/beltbuilder/ but I'm not sure which one would be the right ones to get.