The 2 sizes with generic artwork are:
Small - 600mm x 850mm
Large - 850mm x 1500mm
There is also an option for a custom mat, they don't stock these. NB: there is a turn-around time of around 2 - 3 weeks on custom mats.
If you are interested in the mats, please contact the supplier directly:
JMacgraphics
Contact email: justin@jmacgraphics.com.au