How do I know what is the right bottom bracket for my old road bike?
In general, BB lengths follow the specification of the crank. Thus if you have a look at this site, it will state the BB length for this particular crank
http://velobase.com/ViewComponent.aspx?ID=5d1f43ed-82ec-48d0-b6e2-65f92ecdfcbe

If your crank specifications are not available, then i'd start by measuring the current chainline as per Sheldon Brown (genuflect now)
http://sheldonbrown.com/chainline.html

Then measure your BB spindle length and adjust accordingly. 

Also a good start is to know what crank you are using. If its Shimano, its likely using British ISO type BB. If its an Italian or French branded crank.. and depending on its age etc.. it could be using more 'exotic' BBs

You can do a search for Sheldon Brown Bottom bracket for info on that.