I agree with Kronnie, let her pick it out. Just make sure that if you propose this idea you're mentally prepared for
anything she chooses. She may find something that looks interesting because of its shape or special features that just happens to be bigger than you...or she may even choose something smaller. You just never know. There's such a wide variety of vibes out there because not every woman wants the same thing in a sex toy. And even if the toy was bigger than your dick, that doesn't mean she's going to prefer it to you...no way! Absolutely nothing compares to the warm throbbing goodness of a real cock...attached to a body that can do a helluva lot more than a toy ever could

Unless you've got a fairly sizeable and high-class adult store nearby, I'd suggest doing your shopping online. A lot of adult stores carry cheap-o products that break in no-time flat. There are certain names, and materials, you can trust for durability and functionality (I've had good luck with Fun Factory and Doc Johnson products). There are sites that rate toys, too.