Clematis vary a lot in size. They could be anywhere from 3ft-12ft (for that large flower type). I think you could be safe with a 6ish ft trellis.

If you have the tag with the name on it that will give you size info for that variety.

My experience is the climbing ones, not the ground sprawling ones, can all get over 10' tall in the right conditions. I agree a 6' trellis would most likely be fine.

it depends if its the type the heaps upon itself over teh years.. gaining height.. or one the you have to cut down every season ...

just buy the trellis for its appearance and aesthetic alone.. and then add the flower as a bonus.. that way you are happy with the trellis.. regardless if whether the flower covers it ...

in other words.. the trellis is an architectural variable... not a plant variable ...

and yes.. you need to buy from quality vendors to know what you are buying... so as to fit a precise need ... clems are very variable in the trade once you get down the supply chain.. the usual label simply says.. clem .. and not much else .. though it does look like the old standard jackmani.. if that one is still it he trade ...

https://duckduckgo.com/?q=clematis+jackmani&t=ffcm&iax=images&ia=images

the rents had one on the corner of the awning for decades .. and over said time. some years it covered the whole end of the awning.. and other years ... barely straggling up the post... never really figured out why.. year after year.. it varied .. of course it was right next to the downspout... and winter water could have been an issue.. or salt off the cars on the driveway.. crikey.. now that i think about it.. im surprised it survived at all.. lol ..

ken

I don't know a lot of Clematis by sight but I do know that isn't C jackmanii.

With the exception of the late winter or early spring blooming varieties, ALL clematis benefit from annual pruning. And I doubt that it is jackmanii - that is a late season bloomer and gets massive compared to many other varieties - 4-5 meters is not uncommon.

Post any additional questions in the Clematis forum - there are a lot of very experienced clematis growers there who can offer excellent advice and guidance.