Girl new to motocross. Looking for the right size bike to ride with the boys. HELP!!?

I want to ride motocross. I am a slim girl about 5'7" and i dont know what kind of bike to even look for. There are numbers that describe the bike size. What do the numbers mean? I ride ATV's and I ride big horses and i can completely control both. I'm pretty sure i can control a bigger bike but I... show more I want to ride motocross. I am a slim girl about 5'7" and i dont know what kind of bike to even look for. There are numbers that describe the bike size. What do the numbers mean? I ride ATV's and I ride big horses and i can completely control both. I'm pretty sure i can control a bigger bike but I have no idea what to look for. Can someone help me with bike sizes or show me where to look so i can learn about different bikes. i'm probably going to buy a new bike so please dont try to sell me your bike. I dont want a bike that is just for beginners because i learn fast and i dont want to have to sell it when i want to start competing. How much shoud I pay for a bike? Can I buy a new one or would it be better to buy a used bike? Are there any brands or models that are better? Any tips appreciated. Thanx! ; )
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