Do you know what size is best for you? If you have never tried anal play before, it is best to start small. I suggest that you go to the Anal Toys section of the site, refine it to just Plugs and the size range you're comfortable with, and organize the results to show the products by rating. Then you can check out reviews on products that have been well-received.
I can't recommend any smaller plugs myself (though I would suggest that you start with a silicone plug), but really no one will know what's best for you other than, well, you! I hope you find something that works, best of luck!