I'd have to take

door number 1. 

Actually, I don't know for sure but recently put an accessory drawer under my paint booth and purchased glides based upon dimension A and it worked fine. If I was trying to use the maximum depth possible I probably would go to Home Depot with a rule and take some measurements to be sure.

A

I searched for 24 inch drawer slides and then looked at the specs on one.  For example one 24 inch slide had a "travel" of 19 1/2 inches.

Full extension available

You can buy 'full extension' drawer slides that will slide out 24" (dimension A).  This example from Home Depot.

As above

I used the 16" version that HVT Dave linked to. Worked great on my funky homemade drawer. They also have a good hold position when closed.

Thank you everyone

All these answers are just what I needed.

Roller versus Ball

I do believe you are referring to ball bearing slides. I do not recall ever encountering a roller bearing slide, although they might exist.

For an example discussion on this subject I offer:

https://www.nbcorporation.com/difference-roller-bearing-ball-bearing/

There are alternatives that may be more economical - a design that uses a hard plastic wheel in a trough rather than the more expensive ball bearing slides. The wheel may use a ball bearing for lower friction (the principle property of ball bearings). Example of the less expensive solution:

https://www.homedepot.com/p/Everbilt-22-in-Self-Closing-Bottom-Mount-Drawer-Slide-Set-D68822E-W-W/302981211

....

Vidmar tool boxes/cabinets have metal roller bearings.  A similar brand has plastic roller bearings.

Dimension B.  I just went

Dimension B.  I just went through this situation building a vanity.

Mike.