We have one in a plastic troughdimensions about 8 inches by 8 inches by
20inches. The cleamtis is planted in the trough in about 7 inches of
moisture retentive coir compost. We have then put 5 inch pots of fern
on top of the compost surface to stop evaporation. I water every day
and feed about twice a week. The plant has climbed up a a 6 foot
trellis and is now running along the washing line. It had covered
about 5 foot of line and now I have turned it back on itself so it
doesn't take up the whole line. It is now winding back along the line
and looking really strong. It is an autumn flowering clematis and
should be coming in to flower soon. I am expecting it to look fairly
spectacular when it does flower.

So to summarize, the size of your container probably will not matter
too much as long as you water and feed regularly. Go for a similar
volume that I am using and you should probably be OK.