Normal practice for machine zero is X and Y fully negative and Z fully positive.
That however does not mean you actually have to home to those positions, it may be more convenient to home to the opposite ends of travel.
For example you may have a knee mill so having the Y axis towards you when homed will make more sense as it means you can load the vice/table easier. If you homed that way it would mean Y was fully positive when homed, you can however still have Mach set the machine coords up as per normal practice.
What you would do in that situation is set a Home Off value equal to the distance the Y is away from fully negative. Then when you home the Y would be set to that value instead of Zero and Mach would still know exactly where the machine zero was.
Regarding the roadrunner, there are no units called in the code but it is written in Imperial units, it is approx 7inch by 6inch.
Hood