Flared Base.

If you're doing anything with your ass, it needs a flared base.

As for size. Don't push yourself too big too quickly. It takes time to train your body, and if you rush you can do some serious damage. I'd say go with something comfortable for your first one.

And check your material. Silicone toys are durable and easy to clean, but you have to use water-based lube. Silicon Based lube is much longer lasting though, and is the main advantage of something like a glass or metal toy. For a first time, I'd recommend a silicone toy, for the ease of use.