you just need to give the Virgin mobile team a call on 789 or 0345 6000 789 & request a new SIM of the correct size.
It will cost 99p standard post or £2.99 for next day delivery (Christmas post though will hold things up).
Or you could pop into your nearest Virgin store.
Or do as I did & just cut your present SIM down to size. - I know it seems like a scary thing to do but it's actually dead easy. There are plenty of "how to" videos on Youtube. Have a go first with an old/spare SIM if you have one.