Well, what is your "size" bud? Are you very long? Shorter, but very thick? Personally, I'm just about seven inches long, and slightly thicker than most. My girlfriend claims I have the "perfect dick". I'm long enough to hit the back of her vagina, but in the right position, I have gone too deep occasionally and hurt her. However, she finds my thickness just right. Her previous bf was not as long as me but WAY thicker. Although she was with him for two years, she has told me that sex was never enjoyable. It always hurt, every time.
It may be my gf's personal preference, but perhaps - if you are very thick - you're wife is having a hard time adjusting to you. My suggestion would be to enter her, and go very slow. Very slow sex, and even stopping with your dick buried in her can be very enjoyable. This will help her vag stretch to you. When she's comfortable, pump away and ram her home.
Remember, the vag canal is only around 5 inches long. Although it will stetch to accomodate us more hung guys, sometimes we're just to long. In that case, you're just nit going to be able to bury your while stick in her, and that's about it. You can't put 5 gallons in a 3 gallon bucket, ya know?