Brushwood lets you search by general size (small, medium, large) as well as a number of other characteristics.
http://www.gardenvines.com/shop/search/

I usually have a window with Clematis on the Web open on my desktop when I am looking at buying clematis online so that I can check out what they have to say about clematis I am looking at buying. If a clematis has a multiword name and you are searching by name, only use one word as CotW's seach engine doesn't seem to like spaces.

Here is a link that might be useful: Clematis on the Web search page

Thank you Babs! Just what I was looking for. Now if I can successfully plant these in the fall in my zone ...

I often plant clematis in the fall here in NH. From early September in a cooler summer season like this until after Thanksgiving in a warm fall. If it's late in the season I mulch it well so that it won't have frost heave problems. Fall's warm soils and cooler air make planting and transplanting much easier on the plants than spring with cold soils and often warm air IME.

downeast, Klehms also has a nice, more unusual clem offering, and limited enough that scrolling through their page of photos is pretty easy.
mindy