How big a peep you need is dependent on several factors. Your draw length, the A2A of the bow, both of which affect string angle, which in turn determines how far the peep is from your eye. The farther away it is the bigger the peep needs to be.
Also, how big is the pin guard on your sight? If you bracket it with your peep then you have to consider it.
All said, there is no cut and dried answer. Because of all these things each person has to figure out what fits their style. And as brushrat said, you can use a smaller one for target.3D to helps with better accuracy and then pop in a big one for hunting where pinpoint accuracy is not necessary, but getting more light through is a major concern.
Example of this is that I personally use 1/8-3/16" on my 3D bow and 5/16" Meta for hunting.
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