Concrete Floor expantion dimentions?

I generally make it eight to ten feet.  I recall being around some heavy concrete stuff my uncles were doing in the '50s, and that that was what they use.  

But I vary it slightly, so that all the sections are the same width: usually if I were to make it, say exactly eight feet, I might have three feet left over for a final section, or ten feet, I might have seven feet left, etc.  So I try to stick between those two and find a width where I make them all the same width.  I realize that is not how it always worked out int he real world, but it looks better on my layout.