A plectrum is a small piece of plastic or even metal, usually in a triangular shape, used to pick the strings of a guitar. These come in all sizes and shapes. These are either hard or soft, thick or thin and small, medium or large, or in any combination of above.
It would be good if one uses a hard large plectrum for an acoustic guitar as the strings need more force to sound. A soft large one for electric guitars suffices. Smaller plectrums are better suited to people with smaller hands. Thick plectrums are generally harder than the thin ones unless you are using a thin metal pick.
But, of course, if your hands are large, you would not be comfortable using a small plectrum. So try out some and get the one that suits you best.
Comfort is what one has to keep in mind here.
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