sorry I left work yesterday so didn't have time to reply .
A nano sim is smaller than the mirco sim . Very tiny to be honest.
If the old sim card fits then fire away. The sim will work . the sims are sent in case the sim card you had was an old sim card where it cannot be made smaller into a nano sim. Virgin obviously want you to be able to use the new phone and sim so its sent just in case it doesn't fit
put the old sim in and give it a whirl
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Virgin Mobile Staff , However, All opinions are my own