Ok. So there's nothing that can be done about the size of your mouth. (It doesn't look that small to me). Bu I'll bet you already know the basic rules:
1. No teeth, unless he asks you for it. Some guys will cover their teeth with their lips and then close their mouth on your dick while they do oral. This is very enjoyable for the top, but just be sure you don't bite the inside of your own lips while you do this. You don't want any cuts inside there.
2. The trick is to use your tongue constantly. Remember, licking a guys shaft for ages is going to make him lose interest. The areas he is most sensitive are on and just under the glans. Rather use your tongue on and just around the head of his dick. You know where you are most sensitive on your dick. That's where you should concentrate your tongue action on his dick.
3. If he starts pumping your face, try pressing your tongue against the roof of your mouth, folding the sides around his shaft, while simultaneously moving it forward and backward in the opposite direction to his thrusts. It will make him feel as if he is pumping deeper than he really is. If you can, try to press your lips firmly over dis dick as he thrusts (without biting inside of your own lips - remember? No cuts). A lot of guys will cum when you do this. If you notice he is getting close, don't let him cum in your mouth.
4. Another trick is to place his glans in your mouth and to massage it by using your tongue in circular movements. (Think of a washing machine). The trick is to use the upper side of your tongue against his glans at all times. You can't give him any pleasure by running the smooth underside of your tongue over the upper side of his dickhead, like so many guys seem to do.
5. Some guys love pressing their dicks all the way into your mouth and then they pull your head against them for as long as possible. The trick is to relax and breathe through your nose, otherwise you'll gag. If he presses his dick against your throat for a few seconds, there's not much you can do. But if you really want to impress him, now is the time. It is time to massage his frenum with the tip of your tongue with all the frenzy you can muster. Not all guys are able to fold their tongues back towards his dickhead, but most of us are able to do it. Trust me, his eyes will gaze upon you with a new respect. If the tip of your tongue can't reach that far back, then just massage the tip of his dick with the back part of your tongue.
6. The most important thing is to see how he reacts. Listen to his breathing and his moans. Be aware of which moves make his dick rock solid and which ones make it go a little softer.
7. Finally, if you have cuts in your mouth or lips, avoid oral sex. If he has cuts or suspicious marks on his dick, avoid sex with him. If his dick smells, avoid him, he may have an infection. Bla, bla, bla....