The arbor of the trim saw attachment is 15 mm.

From the manual:
BLADE SELECTION

Your circular saw is designed for use with 3-3/8 inch (86 mm) diameter blades that have a 15 mm diameter bore. Blades must be rated for 3000 RPM operation (or higher).

NOTE: This saw is designed to cut wood and plastic. Do not use masonry or metal cutting blades. DO NOT use any abrasive wheels.