To get to the PHP settings, go to the left Virtualmin menu and select "Services". Choose from the sub-menu "PHP Configuration". There you have a menu with all necessary PHP settings you may need.
The most important setting is your "Resource Limits". There you can set your different maximum values set in megabytes, if not stated otherwise. If you use your server/domain for a WordPress installation, you will run into a cap regarding your maximum size of media files you can upload. You can change that under "Maximum file upload size". Currently it is set for 2M (meaning 2 MB). Click "Save" in the end, when you made all changes you wanted.