Rollers come in all different sizes, but the most common size is about two to three inches wide. There are also mini rollers that are only an inch or so wide. The width of the roller you choose will depend on the size of the area you're painting and your personal preferences.
To load a roller from a paint tray, first pour some paint into the tray. Then dip the roller into the paint, making sure to fully saturated it. Once the roller is loaded with paint, roll it out onto a piece of cardboard or newspaper to remove any excess paint.