This would be specific to where you are located in the world. In my local area there are 40' galvanizing baths. If the beam is longer than the bath it can be "doubled-dipped" however this does leave a noticeable mark on the member which the architect may not be too pleased with.
If the architect isn't happy with the line visible from double dipping than the beam will require additional splicing.
No matter how big the galvanizing bath is, there will always be someone who wants to galvanize a larger piece of steel.
Most galvanizers list their vat sizes on their website. If you are only dipping one beam at a time you should be fine about anywhere. It is usually the width and depth of the tank that vary greatly. I don't think I've seen a major galvanizer with less than a 40' long tank.
I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.
Call the galvanizer....there is a chance they can dip a 60' beam in a 40' tank if they swing it right.
Only way to know is to call, most of them are very helpful since they want the work.
