Most masks are made large so they can fit the largest heads one thing other than foam is stuffing the top of the head and back head area with plastic shopping bags you might have to use Gorilla tape duck tape. The other thing you can try is stuff a ski hat or beanie hat with the shopping bags and the put the hat on then the mask. You can also try safety pins in the back to draw it tighter but then you might need a wig to hide that. The most important thing is your eyes line up with the mask if the nose and mouth area are too big or long it shouldn't matter as long as you can see out of the mask. Not knowing what type of mask ( plastic, latex, silicone, ) you are really talking about the last desperate option is to cut the mask with an Xacto blade and then use liquid latex or tape on the back side to piece it together. If you had a pictur of the mask or you wearing the mask so we can see what the issue is you can probably get some better solutions.
