As per the Canada Revenue Agency (CRA), you can use either square metres or the number of rooms you are renting in the building. If you use the number of rooms, then it’s based on the total number of rooms in the house, not just bedrooms. Ex: Your house has 10 rooms and you’re renting out one, so you can claim 1/10th of the expense.
The above applies for expenses you have for the whole house. Any expense that’s just for the rented room you can claim in full.
You read more about rental expenses on the Canada Revenue Agency (CRA)’s webpage: Rental Income - Expenses - Personal Portion