According to Saga magazine, ramblers usually only flower once and climbers have repeat flushes of flowers but they suggest growing the two together to get blooms until September. My climber is still in bloom now but heading ever upwards towards the roof despite constant yearly pruning!

Take a look at www.classicroses.co.uk - you can select roses by lots of characteristics - try height 1m-1.9m or "suitable as a small climber"
For a 6ft obelisk you might find some of the taller shrub roses (Canary Bird has a nice loose structure) are better than a climber. I certainly wouldn't go for a rambler on an obelisk.

Ramblers are probably too vigorous for a 6' obelisk and as has been mentioned they only flower once a year, in June.

Some climbers will also be too vigorous for that height, but there are plenty that only grow from 6' - 9', are repeat flowering so would be more suitable. A few of these suggestions are: Dublin Bay (scarlet), Chris (rich yellow) Dancing Queen (medium pink), Schoolgirl (light orange) and White Cloud (white).

As said, ramblers are too vigorous and tend to flower once and in clusters. I have Canary Bird fanned against a fence and although the yellow single flowers are beautiful, it tends to flower only once. Dublin Bay would be my choice also as I have one growing over a low arch as it is barely a climber.