The vagina is naturally elastic. Chances are, you will find it can comfortably accommodate even the largest human penis size, even fully-erect, even in cases where at first blush - the dimensions may seem frightening. As a preparatory measure to ensure comfort and enjoyment (to say nothing of peace of mind), perhaps nothing could be better than sexual arousal.
All the above presupposes consent. I'd like to think that goes without saying, but that may be my typical patriarchal guy style rearing its ugly head yet again, so I err here on the side of caution, if any: guys?
Consent comes first. Then her, then you.
Ladies, please feel free to skip over any chauvinism AKA chivalry implicit in that last bit, with your characteristic and stereotypical grace. Thanks!