Californian lilac 'Trewithen Blue'

I bought this plant at B&Q and the label said it grows to 2.8 x 1.8m.  It was a large plant and was 'pre trained' up a little trellis which appealed to me, as I want to grow it against a 2 m fence. 

Got home with the huge thing in the boot, and googled  'Trewithen Blue'. RHS say it grows to 4-8m 

Should I take it back and swap for a more normal sized Ceanothus(I think they had Concha and Skylark in stock)?  

Or will I be able to happily grow this thing along my fence with a yearly prune?  I do quite like the plant as it has bigger leaves than the other Ceanothus I've seen, but I also want an easy garden to maintain!

Thanks,
Adrian