Jackmanii is about a 12' vine so that obelisk won't work for it long term. It takes three years for Clematis to reach their full size. Size varies with climate and growing season too.

It should be cut back to about six inches every year but that won't keep it smaller.

There are smaller Clematis that would work great for that obelisk but Jackmanii isn't one of them.

So, what size and type would you suggest. The plant is about 6 feet away from any wall, so a fan type wouldn't work. Also what type clematis would work with the 4' trellis? Also, are you supposed to cut them back every year? I knew you were the first year, which I did, but I didn't know about every year. Hope you don't mind all the questions, but I really like clematis and would like to have several varieties. I'm in the learning stage.

Jackmanii is a type III which are supposed to be cut back every year. They bloom on new growth and if you don't cut them back you'll end up with bare brown stems and flowers way up in the air.

It is a 12' vine more or less and needs a trellis/structure or tree/shrub that can support that size.

The link below is to a search result where I put two meters/six feet as the maxium height.

I have Piilu on a 4' obelisk and can't say enough about how well it does. I do prune it as a III rather than a II.

Here is a link that might be useful: shorter Clemmies