There is opportunity to tear something if you don't know what you are doing and end up embarrassed in the ER.
As a rule of thumb, the bigger the cock, the more foreplay to stretch your hole prior to penetration. This is something your partner should know unless he's inexperienced too. He can't just jam it in there like in a porno.
My suggestion is to practice on your own with similar sized toys, use lots of lube but also remember to reapply regularly. And most importantly, start in positions where you are in total control of the progression like cowboy until you have more experience.
Unless you are thoroughly cleaning yourself out, you may want to consider not going balls deep because he will probably drag poop from your colon.
If you feel him hitting the back of your rectum, tell him go shallower or change your position so he can pass through your "second hole". Normally positions with your knees to your chest will make it a bit easier but NEVER force anything.