Saw blades with small arbor washers are prone to flutter in use.
One of the best-kept secrets in woodworking is that arbor washers (or stiffening plates) should be at least 1/3 the diameter of the blade used. They can be more than 1/3 the diameter if depth of cut is not reduced unacceptably. A blade is subject to substantial stress in use, and anything that increases stability will improve the cut, especially with thin-kerf blades.
The 3 1/2" diameter size is for 8", 9" and 10" saw blades; the 5" is for 10" and 12" blades.
Sold by the pair, each side is 1/4" thick and hollow ground to a 3/8" wide rim for good seating. 5/8" arbor.