Are you sure about that? I have 10 mgs CJC-1295 and 10 mgs GHRP-2, and these vials held only 2 cc.
I'm assuming you are dosing at the microgram level...? If that's the case, (and if you vials can really fit 5 cc), then add 5 cc.
Whatever you do, the important thing is to keep track of how much water/volume you add! Lose track of that and your F*d; but if you know how many mgs of compound you started with, and how much water you added, you can easily figure out dose.