My Konigskind clematis has never gotten much taller than 3 or so feet tall. It is one that is routinely recommended for pot culture because of its short nature.

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The Raymond Evison Patio Collection of clematis are also ones purported to being good for containers and not being as taller as others.

Here is a link that might be useful: Raymond Evison's Patio Clematis

Some of the Estonian hybrids grow short and have the bonus for us of being super cold hardy. Some of the most available are Piluu, Miniseelik, & Marmouri. My spelling may be completely off, don't even want to attempt to spell the name of the breeder. Starts off Kvis---

Marie in ME

Do your mailman a favor and put your short Clematis in a pot on your deck or porch.Flowers on the mailbox,although beautiful, draw bees.Coming from a long line of postal workers the two most dreaded things on a mail route are dogs and bees.Your mailman will thank you.
Nancy

Nancy, in my garden I rarely see bees on the type II flowers. They tend to be drawn more to the smaller flowers on the type IIIs.

Oops, I have flowers all around my mailbox though not Clematis. I'm allergic to bee stings but I don't let bees worry me.

Somewhere under here is my mailbox. Have no clue what group 3 this clematis is - doesn't matter - she climbed up and made her home on the mailbox last summer.

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Seeds...how beautiful!!..Jeanne