I had an oboe reed handmade for me and the reed itself is very good and responsive, but the cork of the reed is too long, essentially making everything I play sound flat, even when pushed in all the way. I wasn't sure at first if it was too long, but comparing it to the reeds the other players in my section use, there is a noticeable difference. I have no experience with reed making (which is why I had it hand made for me in the first place) so I was wondering if there was any way I could make it shorter myself. Considering I have no experience in reed making, I should probably get into it if I'll be playing the Oboe for the rest of my life, and I suppose this would be a good start.
You can cut the staple shorter, but it depends on the staple. If the staple is the type that doesn't have a rolled or fluted edge at the cork end, you can shorten it with some basic hand tools.
Using a sharp knife (such a an x-acto hobby knife) or razor blade, you cut the cork around the staple pipe at the new length distance. After marking the cut you can roll the staple on a hard surface while pressing with the blade to get a clean edge.
Once the cut is made, remove the cork off of the staple from the cut to the waste end. At this point you will need a fine tooth metal saw such as a jeweler's saw or metal cutting razor saw (I believe x-acto makes one of these also). Using the saw, carefully cut the staple pipe flush with your cork cut.
You then need to clean up the edge, removing any cutting burrs. This may be done with a file or scraper. The end then can be sanded smooth by placing some sandpaper on a flat surface and running the end across it.
If you haven't done this kind of work before, you should practice on some spent reeds first before attempting on your good reed.
Since it is a hand made reed, you should consider asking the maker to do the shortening for you instead of attempting it yourself.
Reed making can be entertaining to do, I found it similar to tying fishing flys, and yes, learning to make your own reeds is the best way to make sure you get exactly the reed you like.
Resizing your reed when you have no experience with reed making will probably be hard and the chances of you breaking the reed are quite high. You can either cut the bottom as Alphonso Balvenie described or you can cut the top.
Now there is a chance that you can just cut a tiny bit off the top and have the reed still working. If you want to get into reed making you could use this reed as your starting point.
As a first step I recommend you take some very close pictures of the original reed. Try to get as many details as you can so you can mimic the woodwork when you try it yourself.
What you want to do first is make the reed wet. Let it soak up water for about 5-10 minutes. This makes it soft and reduces the chances of the wood breaking. Then you need a sharp knife. Since you're not familiar with reed making I assume that you don't own a knife specifically for reeds. Don't worry, a common cutter knife does the job too if you're careful. Many professional reed makers even use special reed guillotines for cutting the top. These are quite expensive though and by no means a necessity. It just makes the job easier and cleaner (http://www.reedsnstuff.com/en/Service/oxid/Reed-Guillotine.html) So if your reed has softened up a bit get your cutter ready and cut a tiny bit off the top. Use a flat surface to cut it on and press the reed together so you cut both sides evenly. Cut a straight line and do it in one go. After you're done see if you can still play it. If all goes well you can repeat this until the sound is fixed.
But chances are that it will get hard to play at some point. That is usually because the wood at the top is now too thick. Now again for the next step a special knife would be recommended but I have heard of people who literally used a normal cutter to make all their reeds so I guess you can use that.
Also before you start I recommend you check out some tutorials on reed making just so you get to know the basics and see how it's done (like how to hold the knife and the reed) You have to scrape off little amounts of wood near the top of the reed. Do not use too much pressure as you risk ripping the top off. To stabilize it you should put a small, flat metal piece between the two reeds (something like this http://www.reedsnstuff.com/Oboe/Rohrbauzubehoer/Schabezungen/).
Some final thoughts. Reed making is a craft and everyone uses a slightly different technique. You need a lot of practice to find what suits you best but in the end it is worth it. A good teacher can help you out a lot. You could try to contact whoever made your reed and ask him if he could teach you. If you're not into that you could also just contact him and ask if he could use smaller cork pieces or shape the reed a bit smaller.