As far as child's bikes go, there isn't too much hassle with respects to how big and strong the cranks are. In general the cranks are designed to suit every rider as an average. As the riders are children, these averages seem to work well, and the strength is not an issue when it comes to the type of riding the children will be doing.
Adults on the other hand need to take more care with respects to what BMX Cranks they choose to put on their bike. Depending on if they wish to sit down on their seat or not, the size of the cranks can determine the difficulty in pedaling.
If they are too small there will be a large amount of effort required to turn the sprocket. If they are too large, rotating the cranks can be awkward.
In order to combat this, when you are deciding on what BMX Cranks to buy for your bike, you should be sure to check out the dimensions of the cranks including the length. Measure yourself or ask someone at a bike shop to ensure that the cranks will not be difficult to ride with. Furthermore, if you wish to be doing extreme stunts on your bike, the strength is also an issue. Be sure to ask a bike shop or look at forums to see if the BMX Cranks you are purchasing will be strong enough. Generally you can be pretty satisfied in this area by using the rule of thumb: the more it costs the stronger it is.
I hope this has helped you decide which BMX Cranks will be suited for you.
Ways To Have The Right BMX Cranks