Don't try to have sex with someone just because you find them physically attractive since it sets the expectation that they're doing it for the same reasons and amplifies concern about anything you consider to be a physical flaw about yourself. Stop putting so much thought into sex. Start focusing on the person and having fun and getting to know them. Date around without pushing yourself to make moves until you click with someone so well that you don't think about making a move. It just happens.
When you find someone with a matching personality/background that's as interested in you as you are in them you will sync up naturally. This reaction is built into you almost biologically. People begin to mirror the person they are attracted to. It starts slow with a touch, like bumping arms or touching legs, the other doesn't pull back to give you space because they want to keep touching you. When attraction is mutually solid like that both brains will signal endorphin release when you touch that will make you both want to touch each other more.
When it happens this way instead of being forced things that are feeling like giant walls now will naturally go away. When someone's really interested in you they will show it; they'll be more interested in talking with you than fiddling with a cell phone or replying to text messages. If you suggest doing something that puts you alone with the other person they'll be pleased and happy to focus their attention on you. Simply focus your attention on them and keep the conversation flowing.
Genuine attraction is more than just physical, it's a meshing of common interest and personality. Conversation should get easier and more fluid as you go along. This is where you can really gauge non physical attraction, the things they talk about when you ask normal questions about them should be interesting to you enough to create natural discussion. Best of all is when you find relating individual experiences you've both had previously that allows you to talk about things you enjoy at a level that's deeper than the average persons knowledge of whatever that shared experience may be. If you can have a conversation with so much deep subject jargon that other people around you in a public place couldn't follow along then you're probably meshing personalities pretty significantly. Do that 2 or 3 more times on a first date and you're probably dry humping each other the first alone time you get.
If you're having penis size anxiety when getting into bed for the first time I would suggest guiding her hand(s) to feel your penis before you show it to her (obviously once you're already in a setting to have sex). Since you've been escalating touch at this point they should happily follow your lead down. Your partner should give you the same escalating touch there (relatively to the previous touching/rubbing of your bodies) as every where else. Once you get this far you should be feeling pretty biologically reassured that your partner enjoys you and your equipment as your brain will be flooding pleasure pathways. If you need any reassurance you're partner is feeling the same way look in their eyes when you're somewhere with decently good lighting. Pupils dilate when you're looking at someone you like so theirs will be larger than you would expect the average persons to be in that situations lighting.
Having sex first time with someone is always a bit awkward as you dont know them sexually yet but when things are really mutual both peoples brains will be flooded with so much 'feel good' that what would be normally an unpleasant anxiety with turns into the butter flies in the stomach. So if things don't go perfectly the first time it won't matter as they will still have felt so good they want to come back again. Once you get there the sex gets better as people get comfortable with their new partner. This is what drives horribly cheesy or bad love poetry (and even the good stuff) is that the receiving persons brain will turn what is cringe worthy into something sweet and endearing even if the rest of the world disagrees.
tl;dr: put yourself out there to meet lots of people and date around without trying to have sex, when you find the right person your brain will override the anxiety and sex becomes a natural process.