Clematis vary widely (pun intended)

...wow....
thanks.....

It really does vary quite a bit. There are clematis that grow 30+ feet high others that will cover a fence with one plant, and also ones that will fit happily in a large pot. You can look at do a search to look at photos (though take colors with a grain of salt) or visit Clematis on the Web and search for info including images by typing in the name (or part of a name) of a clem you are considering. Betty Corning is one that is pretty large.

On a 5 foot wide trellis I have 3 clematis, and I usually put 2 per obelisk. You can always start with 2 clematis and your rose, but spacing so that you can add one at each end or in between later on if your chosen ones don't fill your trellis as desired. And since you said you are growing up to a deck, and added note: just be sure you have something small enough in diameter for the leaf stems to wrap around - wooden trellis generally is too big.

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

I actually typed a bunch more information and have no idea why it didn't appear on my post...

A big grower like Polish Spirit would take up most of that space as would Betty Corning, both would look good with that rose.

You could put two if they were smaller growers like Prince Charles and Rouge Cardinal

This may help. Here's Oranges 'n Lemons growing with Polish Spirit and Gravetye Beauty. Venosa Violacea grows in the same area but you can't see it in the picture.Of course the color isn't quite right but it's close.

{{gwi:577253}}

Thanks everyone for all the advice and input; much appreciated!
Hubby and I dug the bed this past weekend; I think I'll go with 2 Daniel Derondas though I'm torn between to vendors: Brushwood or Silver Star.
I've ordered from Brushwood before and was impressed with them. Never ordered from Silver Star.

I have ordered from and been happy with both of these vendors. Brushwood's plants, as you already know, are very good sized in quart pots. Their packing is wonderful, and I don't believe I've ever experienced so much as a damaged leaf on their plants in transit. Silver Star is also very good. Their plants are usually slightly larger than Brushwood's, and because they ship plants grown in gallon sized plants, the shipping will of course cost more. I don't think you could go wrong with either vendor.