Some of the Evison clematis were bred to be short, so if you don't know the suggested height of your specific plants, you might want to check on Clematis on the Web about the height of yours. (Only use one word in a multiword name as it doesn't do well with spaces.) Also, if your plants aren't 3 or 4 years old, you may want to wait a bit as they do require some patience to reach size if the plants didn't start out large.

I don't quite follow this:
"I'm looking at one that is 3' feet high but is 9 ' long."
Are the 3' and 9' for the spiral or pot or clem?

I do think that the shiny coat on new spirals might cause the clematis to slide down the spiral, though I am not sure. A year of weathering would take care of this problem most likely. A single spiral will really crowd the vine, but you could use several spirals set in a ring around the plant if they are tall enough, but if you have some of the taller clematis, even 6'-7' spirals won't be tall enough for the clematis unless there is something else they can climb on or you don't mind a lot of draping downward.

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

This is the grow spiral,
http://www.leevalley.com/US/garden/page.aspx?cat=2,33286&p=46939

The RE clematises I have on the ground have barely put out any height growth to go past 2 ft even though they've all flowered ok and most are even doing a second flowering. I checked the maximum heights on the ones i have abd they did range to 4ft. I put 1 L root sized clematis plants that seemed to have flourished in the same spot. Only Chantilly is putting out lots of new top growth.
Couldn't resist any of the then last and this year ESP since they were 3-5 bucks on clearance and I have to say I think they looked better in the pots.

I was looking for trellis structures that can grow in a pot that were more 3D as opposed to the flat fan structures. Not really any variety where I live.
What size pots should R. Evison clematis plants go in?
There has to be some sort of proportion rule somewhere to determine pot size to trellis height?
Anyone?

OK - that's rather different than what I was envisioning for a spiral trellis. Looks like it will do fine for a shorter clematis.

I am sorry that I can't help much with the pot size since I only grow them in the ground. There is an FAQ question about growing in pots, but it doesn't give specific pot sizes. It does make suggestions about the potting mix to use that might be helpful. I did a search on the main clematis forum page (down at the bottom) for pot* and found a few references to pot size:
Gardengal48, a nursery professional, said 18-22" was a minimum size for a 3 year-old clematis. A couple of others mentioned half whiskey barrels. If you go for the half whiskey barrel, do you have a way to move it into shelter for the winter (like a large dolly) or are you planning to insulate it? I think some clematis will be borderline hardy in an unprotected container in the winter in zone 5b. Unless you have a narrow pot base, I think a 3' trellis shouldn't be a problem as far as stability.

Here is a link that might be useful: Can I grow clematis in containers?

I tried to use copper tubing to spiral Clematis up a tree trunk. Wound it around like that trellis. Didn't work well at all. Going back to wrapping the trunk with a wire grid next year. I'm done messing with the spirals. I would not advise it. Clematis almost always are going to perform better in the ground than a pot. Many of the Evison's are supposed to stay short. Digging them up and putting them in pots is not going to encourage taller growth if that is the case.