Because it is a vine, height is all you will ever find given on mature size :-) Spread will be determined on how it is trained.......for example, one often sees these clems grown over fences in my area. Obviously, the fences do not extend the full vertical height of the vine, so the vine rambles along the fence until it has achieved its optimum size. Multiple stems emerging from the root crown will enable you to spread it out a bit if present but this is a big, vigorous vine and that is usually not required.

I'm sorry but I don't understand your question about the trellis. Vining clematis attach by leaf petioles so the support needs to be the right size - not too big - for the petioles to wind around. To be honest, hardly any of my clems are on any types of trellising and they are all able to vine and/or clamber through and around whatever support system is available (usually another plant) all by themselves.

Thanks gardengal. I understand why you didn't understand my trellis question - it made a lot more sense when I was very tired. :p What I meant to ask might make more sense if I put it in context. I'm considering the clematis as summer shade for a large west-facing wall. It's the end wall of our house and I'd like to be able to cover it efficiently with something green. I'd like them to climb as high as they can (~25 ft), and space several plants to cover the spread we have. Does it sound reasonable to cover a wall this high within a few seasons (perhaps keeping the plants closer so they can focus on vertical growth instead of spreading out)? I hope that makes more sense.