A last but key element in choosing mats is to determine how wide the mat borders should be around your image. Once again the rule of matting to draw your eye to the focus of the picture can be your guide. Too wide and all you see is mat. Too narrow and the mat will seem cheap or unnecessary.
Some professionals have a comfort level with a simple percentage – i.e. 15 - 20% of the smallest dimension of the image should be the mat border. Using this rule an image that is 24” x 30” would have a minimum mat border of 3.6” and a maximum of 4.8” – probably rounded off to 5”. This would provide enough weight around the image without taking away from the picture.
Others are less exacting and often through experience have found certain border sizes to work well with certain image sizes. For example – they always like a 2.5” border on images that are between 8” x 10” and 9’ x 12” and a 3.5” border on images that are larger than 9” x 12” - say up to around 16” x 20”.

Experiment with what works best for you – but keep in mind that your goal is to get the viewer to look at the image – not the mat.
Artwork by Lynn Blaikie - Island Art Publishers - Photograph Unknown
Want to practice matting for free? Try the MatoMatic framing tool at www.matshop.com. You can upload your own picture or choose from one of their gallery images and try different mats and mat combinations, frames and even background wall colors while you design your own projects.