Len,
First you really need to decide on the overall width of your frames, take into account bearings and their overhang etc, once you have that dimension over the 'outside' faces then you need to define your material thickness, once you have that then your frame spacing will be the gap in between.
If you decide on the frame spacing first and then try to make everything else around that then "path of much difficulty you shall have young padwan".
The overall width will be defined by your layout or perceived minimum track radius you will encounter, I've opted for 6' radius and B7 point work. In greater detail, the overall width is determined by the type of bearings you use, the way they fit into the frames and the amount of side play you ultimately want.
In short, start from the outside and work inward, when you have eliminated and measured everything else the dimension you are left with will be your frame spacing.
Addendum; On reflection this is the order of importance for the path of enlightenment,
1: Define which bearings you are going to use and work out how far they stick out from the frame.
2: Define the frame material thickness.
3: Define your side play.
When you have measurements for all three above then your frame width can be worked out