The key indicator is your kid's height. If your measurements fall in two sizes on the chart below. In 90% cases, we will recommend the bigger size, which will last longer. However he/she might not touch the ground at the very beginning, training wheels is needed during this period. Compare the (inseam + foot) length with the saddle height (adjustable).    

And if your little one is not that adventours, or if he/she doesn't know how to ride, then the smaller size will be easier to control.