The dot is a nominal 2 MOA, or 0.0333...degrees. The angle starts with its apex at your retina. The sides of the angle, if you spin them around the centerline between them, trace the sides of a cone whose axis (that centerline) passes through the middle of the red dot and whose sides just kiss its outer edges. The sides keep spreading out as you go further from your eye. At 100 yards they have spread to two inches and change. At 200 yards, they have spread twice that much, and so on.
A simple way to look at it is, the dot covers a two-inch circle at 100 yards. At two hundred yards, where the same target looks half as big in the optic, it will cover a four-inch circle. The further away the target is placed, the smaller it looks, so the more inches that same dot covers.
As to the physical size of the dot, it depends on the design of the optic. If the the field of view is 20 mm wide and your eye is six inches from the focus, and the magnification is 1X, you have about a 450 MOA field of view, so the dot would appear to be a 225th of 20 mm, or about 3.5 thousandths of an inch. That's awfully small, so I assume the field of view is actually smaller, but the product description doesn't say.