What size dirt bike should i buy?

i think pakistan goona try new talent so they will lose

At 5'6 she should get a couple of years out of something like a crf150f, ttr125, klx125. But I would suggest you take her along to a motorbike shop and get her to sit on a few to see how she feels on them. The bikes I have suggested are basic trail bikes and quite good for starting out as they wont be too heavy for her. They are not designed for mx though so are not suitable for jumping or racing. If she was intending to race then she should have either yz85cc, cr85cc, kx85cc or crf150r. But they are race bikes and may put her off riding if she went straight to one of those.