Having a bike that fits you ensures you are safe, fast and comfortable. The BMX race specific bikes have defined sizes.
See the chart below thanks to LUXBMX (a Queensland based online retailer).
Balance Bikes - Mini Wheelers
BMX Racing in Australia has adopted a mini-wheeler class for those that have a need for speed but cant quite handle pedals yet. Training wheels are not allowed on the track for both personal safety and maintaining a safe track surface, balance bikes are the next best thing.
In race conditions the track is closed and the mini-wheelers will race part of the track. Parent are welcome to help their little mini-wheelers navigate the race.