I'm kinda in Mindy's camp. You don't give enough information for anyone to make an informed suggestion. What are your experiences and goals. If it's for clitoral stimulation size can be irrelevant - if you contemplate penetration how experienced are you? If use to regular sexual activity anything up to 1.5" should be OK - but if you like a fuller feeling, have had kids or just like size going up to 2" isn't unreasonable.
DeAnn is also correct to say that starting small is sensible. It isn't likely you'll stop after just one vibe. After you get one and see how it works for you, you can evaluate that experience compared to reviews of toys that look interesting.
Find contributors that seem to share your experience and skill level and read their reviews - its an invaluable tool to find the right toys.