www.BritArt.com
It's possibly a big more 'complete' than you had in mind, but there are some nice ideas that could be incorporated into your site such as the handy reference illustration comparing the size of the painting to a six foot person.
I'm not sure if they handle the 'selling on' of art or if they only deal directly with the artists, but it's worth checking out.
Given the site title, they may also require that the artist be British.
(I plan/hope to be showing/selling some of my own paintings there when I finally get my arse in gear and read the bumph on the site.)