Hi,
if you have the Custom Fields extension installed then you can set max file, max number of files and the max image width/height after creating new Classifieds Publish form scheme and editing the Gallery field.
If you do not have the Custom Fields extension you can customize the file restrictions using the Limit File Upload snippet https://github.com/simpliko/wpadverts-snippets/blob/master/limit-file-uploads/limit-file-uploads.php but note this will require some PHP programming knowledge to customize the limits to your needs.
How to install the snippet you can read here https://github.com/simpliko/wpadverts-snippets/ at the bottom of the page.
Also note that usually your server will have 2MB or 8MB max file size limit that will take precedence over the WPAdverts settings.
