When the user wants to upload a photo, he has the option to select one of his files or take one at that moment from the mobile phone camera. However, mobile phones take photos with too many mb and thus it is impossible to load the photo. It would be very nice if there was a possibility in Osclass to automatically reduce the photo according to the settings made by the administrator. Maybe a plugin could do this.

I've not found way to restrict this in Uppy or get some limits/compression for uploaded photo.

I don't know if it's correct, but the way I solved the problem with large photos - especially from mobile phones - is this: I disabled the original photo reservation and increased the maximum size to 102400KB. I hope this increase in size doesn't cause a problem on the server.

This reduce size required on disk.
Only down size of large photo is that it takes bit more time to process thumbnails and can hit some limits (you need a lot of PHP memory).