I'm sure you'll hear two schools of thought about that.Some say bigger plugs are better since you would have to do less digging to the sides of the hole if the target is off to one of the sides,also it is claimed that it's also better for the turf roots.
Myself,I tend to keep plugs on the smaller size,just enough to find my target.I mostly hunt for coins so I really see no need in digging large plugs several times the size of the coin.If you pin point proper with your detector for the most the target will be near the middle of the hole. or in the plug.At times I have been off a little on pin pointing so I just shave the side of the hole till I get it.
As long as you can dig a neat, 3 sided flip plug,recover your target, back fill the hole and replace the plug and you can't tell that the ground was distrubed,then I would say your good ta go.
That's my two cents and good luck to ya...........................................