I would use a 3" (9cm) pot to pot each plug into - then as they grow, move them on into slightly larger pots. (I'm assuming that they are quite small plugs at the moment). Otherwise, pick a pot that will give you about 10mm (1/2") between the plug and the pot all round - and a bit of extra depth for the roots to grow.
Putting them straight into a large pot will not help them as the roots will not be close enough to the moisture below to draw it up.