For prints that size, I'd recommend mats between 2.5 and 3". For B&W prints only, white mats (such as Bainbridge 8655 or 8634, and Crescent 918 or 3297, or Rising White. For color prints, I recommend pale cream mats, such as Crescent 3293, 2961, 3259, and 1000, as well as Bainbridge 8640, 8656, and 8645, or Rising Cream, Ivory, Antique or Natural. All these pale cream/off white colors are unobtrusive. After all, you want folks to look at your image, not the mat. I also highly recommend not using paper mats. They are cheaper than the rag mats I list, but not archival. They are made of wood fiber, which contains lignin, an acidic compound.
Have fun!
David V. Ward, Ph. D.
www.dvward.com