I third starting smaller--I'm not sure why she would want a toy to practice anal if it's just going to be the same size as your cock. It makes more sense to have a smaller toy so she can practice with that, and then work up to a toy that's either the same size, or slightly smaller than your cock. Then, when you guys try anal out, she'll be comfortable enough with the dildos that it will be a lot easier for your first time.