Small, but not small enough :-) There is a form of Japanese climbing hydrangea, Schizophragma hydrangeiodes 'Platt Dwarf', that grows about half the size of a typical climbing hydrangea but it will still reach 20-30' feet in time. Tends not to be a very heavy bloomer also - I have yet to see one in bloom. Any other selection will rapidly overwhelm a 6' structure.

Climbing hydrangeas are BIG vines and develop a very heavy woody trunk and stem structure once established. They require an equally sturdy and sufficiently large support structure as well. And they do not vine but attach themselves with rootlets along the stems - this type of climbing method does not lend itself to small trellises or smaller wooden supports. They need some expanse of surface - preferrably with some texture to it - for the rootlets to grip and adhere to.

You could of course use your wooden structure as a small vertical 'lift' to give a hydrangea some emphasis, but with a support so small, your essentially growing the hydrangea more as a free-flowing shrub or groundcover, which many gardeners do. Just understand that its size will exceed your expectations - and the wooden structure - in time. And more rapidly than you'd expect.

Also, pruning to maintain a smaller size will result in minimal flowering, if at all.

Mature climbing hydrangea growing 50' up a Douglas fir: