Purple Emperor is a more compact one than the normal varieties and will grow to about 4 or 5 feet tall and about 4 feet wide.

There are other dwarf ones too such as Blue Chip and Lilac Chip.

If you keep on trimming it and trimming it as it grows in early summer I think you will lose the full impact of its flowering. Dont know how big is acceptable to you.

Agree with Inverglen - you'll not have good flowering if you keep them small, better of buying Buddleia 'Buzz' or Blue Chip as mentioned - they get under 3 feet.

However you can cut them back drastically in early spring and they will shoot up again to their full height in a single season, so keeping the taller varieties dwarf isn't really an option as you would have no flowers as the others say.

Thank you all.