Referencing the Ontario Building Code, the maximum size of a shed without a permit is 15 square meters or 161.5 square feet.
According to the Ontario Building Code (220.127.116.11 Requirements for a Building Permit), a shed is exempt from the requirement to obtain a permit under section 8 of the Act and is exempt from compliance with this Code, provided that the shed,
(a) is not more than 15 m² in gross area,
(b) is not more than one storey in building height,
(c) is not attached to a building or any other structure,
(d) is used only for storage purposes ancillary to a principal building on the lot, and
(e) does not have plumbing. The maximum size of a shed without a permit is 15 square meters or 161.5 square feet.
Shed placement must comply with the zoning bylaws of each municipality, and prior to building or placing a shed on your property, consultation with the Zoning and Bylaw department of your area should be done to see what restrictions would apply. These restrictions include but are not limited to the distance between the shed and lot lines, an area that buildings may occupy on your lot, height limitations, etc. Failing to follow the bylaws could result in an order to comply and possible fines.