first, are you sure that your present SIM isn't one of the "push out to size” type that you can make to the size you want?
If not, you will need to give the Virgin mobile team a call back & request a new SIM of the correct size.
Unless they were of the belief that you had a "3 in 1" type. They should have provided you with a new SIM with your upgrade.
You may have to pay for postage: 99p standard or £2.99 for next day. But I would still argue this point and certainly not for the SIM itself, which should have been provided with your new "upgrade" phone.
In the past when dealing with Virgin Mobile I have found that polite persistence usually pays off
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