Question 1 to see how it works.
use appload and load "Multi radio buttons 2 col.
Copy lines 1-6 and paste to command line. You should see the dcl open pick left and right.
Copy lines 7-8 and paste to command line you have the two values.
You would normally include all the code in your code and no need to appload as this is handled by the (load "Multi....)
Question 2 if some lengths are not available then you can use this Multi Radio Buttons.lsp You would use the lst1 and choose a supplier, you then run it again with lst2 but that matches what sizes are available from that manufacturer much easier than greying out sizes.
Check manufacturer and use correct sizes available next.
For code need sizes for each manufacturer.