I have a bonus room over my garage that I enter from my upstairs master bedroom. We use it as a huge closet, however there is no duct/return to it to regulate the temp. in the summer and winter. My bedroom has a cathedral ceiling and, therefore, no way that I can see to run a supply and return from my upstairs unit to the closet. I think I can run the supply and return from the unit in my basement and up the inside of the garage to access the bonus room from below. Im just not sure what's acceptable as far as the length of the run since Id be coming from so far away. Also, does the return and supply vent need to be located a certain distance or oriented in a manner so they're not near each other or not?
The bonus room is about 12' by 20'. I don't necessarily care that it be heated and cooled as well as the rest of the house, Id just like to be able to go out there in the winter without freezing and the summer without sweating. Any thoughts or advice would be appreciated.

By the way the room is finished and insulated

just how far (in feet) will you be needing to run the supply and return
also what you might want to look at is probably the ac/furnace was designed and installed to fit the now heated area,, if you run another leg to another room of 12x20 you might be taking too much away from the other parts of the house,, and do more harm than good.
have you considered maybe a window or through the wall heatpump sized for just the one room only