To add some specifics to Dan Romik's answer: I switched from Matt to Matthew when I took my current job, for a lot of the same reasons as you. I've found it helps to have rehearsed reactions to encourage use of the full name.
People exchange names when they meet for the first time. Usually people repeat the name they hear just to make sure they have it right. Most times, people will pick up on your use of the full name, and check within a few minutes, “Do you go by Matt?” I just say, “No, I go by Matthew.”
But some people will use this chance to establish familiarity and immediately shorten the name. [I know some answerers say that they would never do this, and I believe them. But some people do do this. Sales people, for instance.]
Nip it in the bud like this:
- “Hi, I'm Mike.”
- “Mike, I'm Matthew. Good to meet you.”
- “Matt, how's it going?”
- “It's Matthew, if you don't mind. (smile) And I'm fine.”
Make eye contact there to make sure they got it. But otherwise, don't let the moment hang, just move on.
Sign off your emails the way you prefer to be called. If a person uses the short form to me two emails in a row, they get:
Matthew <-- I prefer Matthew to Matt, if you don't mind”
Note again the phrase if you don't mind. Who would mind calling somebody the way they asked? That smooths things over better than “Don't call me Matt!” (which is essentially what you are asking).
After familiarity has already been established
Sometimes you get to know somebody, or work with them, and you've missed the chance to give your name the way you want it. In that case, a little sidebar like this works:
“Hey, I need to tell you something. I go by Matthew instead of Matt. I'm sorry I didn't tell you earlier, we were just so busy doing X and I didn't get the chance.”
A slightly escalated version of this scenario is when one person in a group hasn't seemed to notice that everybody around you uses the long name, and continues to use the short name. In that case I will say something like, “Hey, do me a favor and call me Matthew, if you don't mind. Everyone calls me Matthew.” It's a bit of an alpha move (if you don't mind take a bit of the edge of do me a favor), but it works.
Add (don't substitute) humor
Dan suggests conveying the whole message in the form of a joke, but I prefer being direct first and pulling back with humor. Although it's for the exact opposite wish, it reminds me of grown men who say, “Mr. Smith is my father; please call me Mike.” That just seems cheesy.
So I might follow up one of the messages above with one of these:
- When I started here I decided that Matthew was my grown-up name. (yes, cheesy, but I've already been direct)
- People don't hear Matt as clearly so sometimes it gets mistaken for Mac or Mike (happens to be true but it's not the only reason)
- My wife likes long names and hates short names. When we met I told her my name was Matthew. Months later she said if I had said my name was Matt she wouldn't have given me the time of day. Since then, I've been Matthew (true up to the “since then” part).
Most of the time, if you just give your name the way you want people to call you, they will call you that. But once in a while you need one of these tools.