Are the 20″ wheel bmx bikes also used by the pros/adults?
Yes, 20″ wheeled BMX are used by adults. I hope you mean BMX racing and not street/vert/freestyle. The big thing you need to pay attention to is the top tube length- the distance between saddle and handlebars- since this is how most BMX bikes are measured. There are 3 typical sizes of 20″ BMX. Junior, Standard, and XL. XL will be too big for you right now but will be needed later. If you are looking at a bike from Walmart or Target or someplace like that they usually only sell standard length. It is not a good idea to buy a bike that is too big now and then “grow into it”. Besides, as you get better you should look forward to a better bike, right? Then you can get a bigger frame.