Can you ask the original owner of the boat/skirt if he was able to fit it on the boat? It's unlikely but possible that they really don't go together and the skirt could be too small.
However, skirts do tend to shrink when they haven't been used and they're dry. Try getting it wet, have a friend help you put it on, and leave it on for a while.
Also, there is a technique to getting a skirt on. What might seem impossible now may be second nature later. Start by hooking the skirt over the cockpit rim behind you, and far enough up the sides that it will hold there-typically at 8 and 4 o'clock positions, but it varies by boat. Don't try to go too far up the sides or you won't have enough slack to work with to get the front on. To get the front on, you really have to drive forward and hook the skirt over the center of the cockpit rim. Once hooked, you should be able to let go and go back and make sure the sides are over the rim.
Sorry if this was already obvious to you, but I wasn't sure.