i've heard it widely said that the average size of an adult male's erect penis is between 4.5"-6". i'm guessing you're around the 4.5 mark then? well you're not a stallion, but you are a normal size. my first sexual partner (well i never measured him, but it is my estimate!) was just over 4.5" erect and it never gave us any problems. i was able to orgasm pretty much every time. as they say, it's not the size of the boat, it's the motion of the ocean. you don't need a big penis to stimulate a woman's clitoris or her g-spot. so unless you are dating really shallow people, you should have no reason to worry in that dept. besides, i've heard the penis can keep growing even up until your mid twenties.
well as for the erection issues, the doctor is always a good place to begin with. whether or not you need to go really depends on how frequently you are having these issues. i suppose it's natural to go through fallow periods if you are under pressure in any way. but it's always worth ruling things out. illnesses like diabetes can often cause erection problems. men can get venous leaks and things too, but as you are young this is unlikely. if the doctor finds no physical problem then he is gonna tell you it's all in your head, so it's up to you to go easy on yourself. relax! take the pressure off yourself.
it's highly likely that if you are running this notion that there is something wrong round and round in your head you'll start believeing yourself. you know what i mean? it's that old story of the viscious cycle. you don't wanna go convincing yourself you have a full-blown problem when it may be nothing at all. you say you were drunk the first time it happened. well you almost have the root of your problem right there! they don't call it brewers droop for nothing. alcohol will kill your erection before it's even started. for sure. i dont know a single guy who can keep it up when drunk. that's totally normal.
it happens to everyone. it's happened to every guy i've been with. don't beat yourself up about it. i'm sure it's just anxiety, mate. relax and take things slow.