What size BMX bike should I get?

Children age six and above should not have any problem cycling on a 20" BMX.  If the toptube is 18" to 20" in length, a child of 5ft 3in in height should still be able to ride on the bike comfortably.

The good part about BMX is that if you opt for a larger size bike and the reach is too far for you at the moment, its handlebar is high and can be moved forward nearer towards you as you sit on the saddle.

Sit on the BMX of your choice, adjust the seatpost so that the tip of your feet touches the floor while you sit on the saddle. Place your hands on the handlebar and feel if it is a comfortable reach for you. Adjust the handlebar forward if it is too far away. Note that your arms should be stretched out comfortably. If your elbows are bent as you hold the handlebar, look for one with a longer toptube length.

In choosing wheel size, make sure that it is easy for you to find the tyres and tubes when replacements are required. Some tyres with size in fractions maybe difficult to find. Examples of such sizes are 20x1 3/8, 20x1 1/8, 24x1 3/8, 24x1 1/8, 26x1 1/8.