Buddleja davidii can grow into a large bush about 15' tall - but if pruned correctly, it can be kept at around 6 - 8'.

if you want something smaller they have a new type called buddleja buzz for pots and beds, but still attract butterflys and just as pretty, ive got two on the way and cant wait :o)

Mine puts on 5ft every year. and it was a smaller variety. it is very compact bush with fat compact flowers.