how and when to prune paulownia tree

It's better to prune any tree, including this one, in late winter or early spring while it's still dormant and not in leaf. Paulownia is a bit of a brute (ours is taller than yours after six years) but you can cut it right down and it will resprout like a shrub, with enormous leaves.
Having read a lot on the Internet about what a wonderful wood it makes, I have gone a bit mad this year and I have almost a hundred Paulownia tomentosa seedlings. (It flowered last year, 2008) Seeds take a while to get going, especially the young seedlings which don't really grow away until June or July. I'm planning planting them out for some quick cover in a field where we had some poplars which have all been felled.
Good luck with your pruning. Keep any large branches you cut down as it makes a surprisingly good firewood.

If you are going to turn it into a coppiced shrub for the elephants ears leaves then the time to prune is in February. You also rub out all but three of the branches which then grow from the base. That way you get really big leaves.