Gino, I'm a beginner myself. I'm sure you'll find this forum useful. I recommend using the search function; you'll find hundreds and hundreds of useful and educational posts.

Choosing which clematis to buy can be quite a daunting task, there are sooo many varieties! I've found the following article particularly useful:

Here is a link that might be useful: easy and showy clematis

as far as hardiness, height of mature plant, pruning etc, I suggest you go to:

Here is a link that might be useful: Clematis On The Web

Any information from Dr. John Howells is good information.

The clematis vines will fall over on themselves when there is no longer support. There are clematis that won't grow very tall, staying around 6 feet. There is a very good book available on the subject by Ray Evison called Clematis for Small Spaces. There is also a collection of clematis, winter hardy and grow around 6 feet called the Kivistik cultivars. Excellent plants. I will have them all eventually. There is a woman who, last year, started her own business sell these lower growing plants online.

I put a link to her website so you can see the Evison and Kivistik collections.

Here is a link that might be useful: Clems for the smaller garden

Here is the whole collection

http://www.prideofplaceplants.com/kivistik-clematis.html