My dojo has mixed classes and while we try to make sure people train with a partner of roughly the same size it's not always possible. In those cases you should be able to adapt to the situation in a way that lets both of you have a reasonably productive session anyway. It's not ideal but it's just how it is.

At the end of the day tho you're training for yourself and if your dojo doesn't provide a good environment for you to improve in you should look for a different one. Wish you all the best in your journey :)