What is the standard size of bridge railings?

Nowadays, there is generally no standard size for bridge railings, and most of them are based on the size of the bridge. Because the bridge railing was originally for safety, now it is for beauty. I make white marble railings. According to the routine, the backboard is not less than 70 cm, and the column of the fence is not more than1.2 m. Now I have done a lot, but they are all done according to the needs of customers.