Here is a link to the Canada Firearms Centre
In this page you can find a reference to two barrel lengths, 470mm (~18.5") and 457mm (~18") being minimum, here is the section. There may be more laws regarding this but this is all I found in a quick search.
"Q. What are the different classes of firearms?
There are three classes of firearms: non-restricted, restricted and prohibited.
Non-restricted firearms are ordinary rifles and shotguns, other than those referred to below.
Restricted firearms include:
- handguns that are not prohibited;
- semi-automatic, centre-fire rifles and shotguns with a barrel shorter than 470 mm (about 18.5 inches);
- rifles and shotguns that can be fired when their overall length has been reduced by folding, telescoping or other means to less than 660 mm (about 26 inches); and
- firearms restricted by Criminal Code Regulations.
Prohibited firearms include:
- handguns with a barrel length of 105 mm (about 4.14 inches) or less and handguns that discharge .25 or .32 calibre ammunition, except for a few specific ones used in International Shooting Union competitions;
- rifles and shotguns that have been altered by sawing or other means so that their barrel length is less than 457 mm (about 18 inches) or their overall length is less than 660 mm (about 26 inches);
- full automatics;
- converted automatics, namely full automatics that have been altered so that they fire only one projectile when the trigger is squeezed; and
- firearms prohibited by Criminal Code Regulations."