Unless there’s some provincial law, RCMP was fucking with you. There is no length law.
What is the maximum blades size one is allowed to carry in Canada?
There is no specific blade size restriction for carry in Canadian law. However, the larger the size of the blade, the more difficult it would be to identify it as a tool for everyday use. For example; if you are hunting, then a large fixed blade knife on your side is generally acceptable. However, carrying the same large fixed blade knife on your side while walking downtown in a major city would be a different matter. If you have specific questions, please contact your local law enforcement agency
There is no law banning the carrying in public knives with sheaths, knives that take both hands to open and any knife with a fixed blade and certain non-prohibited folding knives, assuming they are not carried for a purpose dangerous to public peace or for the purpose of committing a criminal offense.
In Canada there’s no specified blade length that makes it legal or not. The knife’s legality is determined by the purpose of carrying the knife and its design.