Hmmm, hung like a horse,
You're not saying what changed, so it's very difficult to give advice other than the most obvious. On the other hand, a vagina is not something you physically "expand" or "stretch" from the outside - it does it by itself, depending on how sexually excited a woman gets from a psychological standpoint. Now, if she is super-super small and you are super-super huge, unless she is very excited and lubricated, your penis is not going to go in there comfortably. Also, some positions may be uncomfortable for her, due to your size.

My suggestion is that you get with a sex therapist in your area where you live so that you can get some good old-fashioned mutual sensual education. She (the therapist is usually a she) will teach you both relaxation techniques, suggestions for yummy positions, maybe some tantric techniques, etc. The most important thing though is that your wife is RELAXED and really horny. If she is the least bit tense [she can't even have "tense" thoughts!], it's not going to work. Otherwise, there is no way to "stretch a pussy." It doesn't work that way.

Please go to www.aasect.org and look at the list of sex educators and therapists in your area. This is a mechanical/technical issue that can be nipped in the bud before it becomes a huge psychological boulder. And just think of how much fun you can have discovering what you can do, in addition to what you already know.

Remember that the brain is the biggest organ of them all,
the Love Goddess

if you have time to write, we have time time to read whatever reasons that are too long to get into... thanks...  but I feel for you bro...  apparently when you two fooled around before marriage, she should have known that you were hung and it didn't concern her back then?