Which min size, height, width or file size? I just did a quick test with height and width and validation seems to be working for both height and width, logged in using the wp media uploader and logged out using the alternate file upload field.
Are you seeing any JavaScript errors?
Are there any php errors happening during the upload request? Turn on error logging https://codex.wordpress.org/Debugging_in_WordPress
Try deactivating other plugins to see if there are any conflicts. Try switching to a default theme.
