Hello Sam,
Frist of all you are not a sissy. Lots of people are tall and thin and lots of people are short and chubby while others have the perfect shape...............I am not a perfect shape person either. 
You will grow into your tall frame as you age. Trust me on this as I see tons of tall thin boys at school where I work.
Your friend is a bully. Bullies will always exist. It is tough to deal with sometimes, but I learned to rise above it and to distinguish immature name-calling from legitimate criticism. I was not called gay but I was called fatty and other names.
This person that is putting you down is not a good friend and I would really like to see you find friends you find more to your liking.
Remember this person does not get to decide who you are, only you can do that.
Here are some things you can do to combat psychological and verbal bullying:
Ignore the bully and walk away. It's definitely not a coward's response — sometimes it can be harder than losing your temper. Bullies thrive on the reaction they get, and if you walk away you're telling the bully that you just don't care. Sooner or later the bully will probably get bored with trying to bother you. Walk tall and hold your head high. Using this type of body language sends a message that you're not vulnerable.
Hold the anger. Who doesn't want to get really upset with a bully? But that's exactly the response he is trying to get. Bullies want to know they have control over your emotions. If you're in a situation where you have to deal with a bully and you can't walk away with poise, use humor — it can throw the bully off guard. Work out your anger in another way, such as through exercise or writing it down (make sure you tear up any letters or notes you write in anger).
Practice confidence. Practice ways to respond to the bully verbally or through your behavior. Practice feeling good about yourself (even if you have to fake it at first). Soon that confidence will become a part of who you are. 
Take charge of your life. You can't control other people's actions, but you can stay true to yourself. Think about ways to feel your best — and your strongest — so that other kids may give up the teasing. Exercise is one way to feel strong and powerful. (It's a great mood lifter, too!) Learn a martial art or take a class like yoga. Another way to gain confidence is to hone your skills in something like chess, art, music, computers, or writing. Joining a class, club, or gym is a great way to make new friends and feel great about yourself. The confidence you gain will help you ignore the mean kids.
Talk about it. It may help to talk to a guidance counselor, teacher, or friend — anyone who can give you the support you need. Talking can be a good outlet for the fears and frustrations that can build when you're being bullied.
Coming here and talking about your fears was a brave thing to do now know you are normal and that you have many who care about you even when your feeling down and lonely.
If you have questions or need help let us know. Remember a problem shared is a problem halved.
Wishing you the best,
Kitt