How large can the sail maker label be on sails? Or the manufacturer label on other items of equipment? The class rules do not restrict this?

The relevant rules are contrained in the World Sailing Advertising Code, also known as Regulation 20.

Clause 20.7.1 states that the display of manufacturer's and sailmaker's marks is permitted at all times as detailed in Table 2 but not in areas that area reserved for Event Advertising. For radio controlled boats the area reserved for Event Advertising is 40% of the hull length on each side of the hull from the foremost point on the hull. No Event Advertising is permitted on the boom(s), backstay or sails.

20.7.2 tells us a manufacturer's mark may include the name, logo or other identification marks of the designer or manufacturer of the equipment.

20.7.3 tells us that a sailmaker's mark may include the name, logo or other identification marks of a sailmaker or of the sail cloth manufacturer or the pattern or model of the sail.

Table 2 – Manufacturer’s and Sailmaker’s Marks

Radio-controlled boat

Hull  - On each side of the hull, and may include the name or mark of the designer or builder - One mark to fit within a rectangle measuring 15% of hull length x 150mm                     

Spars and Equipment - On each side of spars and on each side of other equipment - One mark not exceeding 50mm length                    

Sails - On each side of sails and kites - One mark to fit within a 50mm diameter circle