Welcome to the forum.
Most suppliers provide an assembly instruction sheet that sometimes has the bushing diameters in them. They are often on a link to a pdf somewhere on the description page others have an instructions page to get it from. You can use it to compare the sizes of different kit tubes and the bushings for them. One of the frustrations is each kit supplier will have their own name for the same kit, the mentioned Sierra being one of them.
There is nothing to stop you from using worn out bushings or smaller than needed as long as they fit the tube diameter from other kits and measure to size using callipers as you work.
Silver mentioned the IAP's list of bushings and tubes. They also have apps for iPhones and Android you can download for a dollar US. It is searchable by typing in a name, diameter or length and a list of all the kits the search matches will pop up letting you make comparisons. It doesn't have every kit or every seller but it is very comprehensive.
For what it is worth I use bushings between centres to get to rough size then finish between centres using digital callipers. A dead centre with a 60º cone in the headstock and a live bearing centre with a 60º cone in the tailstock. No mandrel.
Pete
