Transman bridesmaid.....HELP!!!?

You're combining too many issues at once, which is why you're kind of stalled. And your cousin's wedding is the least important of these. So set it completely aside.

I've never heard someone say they "think" they're trans, nor have I ever heard someone offer to put the transition on hold for 18 months just for a wedding. So I'm not sure what that means.

Just stay focused on figuring this out. Once you know for sure what you are, obviously the path becomes more clear. If you're ready to start a transition, this is when you come out and talk to your cousin. If you decide to stay put (lol) then there's nothing to worry about.

The wedding shouldn't even be factored into any of this. Your gender is far more important than a party.

Why can't a bridesmaid have short hair?

It's trendy to get a short hair cut.

Yes, it may be something you would like to talk to your cousin, not your hair, you can have short hair. A lot of women for many reason have short hair or haircuts that could be guys. Nobody has a right to tell you can not have short hair.

Transman is something you talk to your cousin.

When you are comfortable speaking to your cousin, let her know what you are going through. It's early enough yet in her engagement for her to get used to the idea, so you don't hit her up with a big subject while she's in the screaming hurtle towards her wedding day.

She obviously cares about you, so give her time to absorb it. You may find that she's just fine with you wearing a tux and having a male haircut, or you may find you'd rather put on the brakes for a while, and just get through this big day in a dress.

She deserves to know, and you don't want to spring it on her later. Given how much it would affect her/your family and her day, if you could hold off, it might be easier for all of you - but that's for you to work out, and for her to consider. It's something you can work out as family, but only if she knows. If you can, giving her fewer complications would be a gift.

Having short hair style is just fine for a wedding as a bridesmaid..

There is no such thing as "bridesmaid style". If you want to cut your hair, cut it. You don't need to consult your cousin. If you want to bring up your life changing decision with her because she's someone you trust and feel close to, do so. I don't suggest bringing it up in terms of how it might affect her wedding.

How far along in your transition will you be when she marries 18 months from now? Will you look like a woman? Will you be trying to pass for a man? Will you be taking hormones?

You don't HAVE to say yes, but if you're going to be taking male hormones and have acquired a male name, then the wedding processional will be about YOU and not the bride, not fair to her. Your hair length is really irrelevant.

properly short hair makes you seem much more clean and it makes you seem like you have a good sense of style and long makes a person seem more manly and playful

Medium size hair because it is easy in order to handle and you can still do most of the hairstyles