It varies a lot. For a start there are narrow canals (locks nominal 7' wide) and broad canals (nominal 14' wide). As obviously only a 7' wide boat can go through a narrow lock, most of the bridges on narrow canals will not accomodate broad boats. Apart from those limits, through the general length of the canal the width varies enormously from sections where 2 boats can just pass each other, to places such as Tixall Wide where it is 200' or more wide.
As has been said, there is no standard width for canals. The Manchester ship canal is a canal, but can take huge boats. Narrow canals can only take narrow boats.
Narrow canals tend to be the same width for most of their length.
But where you get to locks, or mooring places, or a canal basin, or where canals meet, they get wider.
There was an argument between 2 people at college about it and they asked me if i could help resolve it, i thought this would be the best place to come, and so it was. :-)