How long should it be?
The HWC is cryptically ambiguous because there's nothing in law to specify a minimum passing distance.
I'm guessing there's nothing stopping you from going right up to whatever the limit is before it'd become a "Wide Vehicle". Can't find a legal definition of that though.
However, if you did put something 7ft wide on the back, I really wouldn't be surprised one of the following things would happen:
1) Someone would lose patience with you and drive past snapping it off whilst mouthing abuse.
2) The Police would pull you over and tell you not to be so inconsiderate.
Something 1m wide (and centred) would be an interesting "theory of big" experiment.