Well for sucking small should be good though my long-time boyfriend had it and it just didn't turn me on. So for me would be anything beside small. But that's just my taste.
For bottoming eh. I don't bottom. Only did for the mentioned above few times. I mean I would love to bottom but I didn't find the right guy and dick. If I stumbled upon a big manly guy I would give it to him. My bf had like 15cm but it was thick like Rocco Steele or something. Gosh. I didn't like it. I want to try with a slim dick very bad. Then I can judge.