Helmet Affairs - The Right Fit
There are lots of reasons to use a full-face helmet, but you won't unless the one you choose fits properly.
No two brands of helmets will fit quite the same, despite being marked with the same sizes.
(It is maybe more important to note that no two heads are shaped the same despite being able to wear similarly sized helmets)
That should tell you that expecting to wear a helmet gifted to you or that happened to come with a bike you buy is a long shot, at best.
Oh, you may find it 'wearable' for a few hundred miles, but if you take a serious trip on your bike and that helmet was not selected for proper fit, you can plan on not being comfortable for long or a desire to end that trip long before you had planned to.
There are three issues involved in proper fit:
Snugness - Your helmet must not be able to twist or 'lift' because  of the wind
Forehead - Your helmet must not leave a red mark/indentation on your forehead
Earholes - Your helmet must not be able to fold or even press constantly on your ears
Issue 1

Proper snugness of a NEW helmet is found when the liner presses firmly (but not tight or uncomfortably) against your cheeks.
There should be no gap between cheeks and liner on a new helmet.

After a few weeks the liner will form fit (because of compression) to the point that you will not notice any tightness against your cheeks, though the liner will remain in gentle contact with both.
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