Growing clematis in a pot

I have a pergola in a corner of the garden, but when it was put in. they didn't leave any open ground around the posts, so there is nothing to plant into.

I am thinking of planting clematis in a couple of pots at the base of the posts so it can climb up and over the pergola, but would like an idea of how large the pots should be to enable good root growth and ensure they don't dry out too quickly, and also not require regular repotting.  Should I go for John Innes Number 3 for the compost?

I have a clematis growing in 30cm pot, but it only attains a height of around 1 metre, so not sure of how big your pot should be, sorry. 

The closer that you can get to a half barrel the better. I have two clematis alpina growing either side of an arch in half barrels, and they have been there for about a decade and thrive. They do still need regular watering in summer as they are about 3m high and bushy with it, plus plenty of food, but seem generally happy.

I'll let you know because I have just planted one today in a tall but very narrow pot in a paved courtyard. I think with regular watering and I suppose some kind of feed when flowering it will be fine. It's right opposite the kitchen window so I will keep a watchful eye whenever at the sink!