i prefer a 1/4 to 1/3 minute dot. i have used 1/8 minute to 1/2 minute. large dots, by there very nature, demand your attention and may distract you from the aim point on the target. one may begin to follow the dot, far too frequently off the target. a smaller dot allows you to focus on your point of aim and bring the dot to it without covering the point or distracting you from it. too small a dot, 1/8 minute, may exacerbate perceived motion and can get lost in low light conditions.
dave imas