too big or too small is not the matter for 1st time experience but the technique/skills and how expert she use her tool that does matter. Too much lube doesn't help either if you don't relax and prepare yourself to pop up your cherry. Dreaming about it is not enough if you don't have the will to fulfill your fantasy.
Does anal always hurt at first? I'm a liar if i say you won't feel nothing. Yes it does (a little) and you going to feel like you want go to toilet but if your pain tolerance is good, then it should be no problem
Half hour session would be too short and stressfull to put you at easy, relax and loosen you up. An hour session is highly recommended. Good luck for your first experience