I am replacing old BB with new 3 1/4" x 9/16" BB. Walls are drywall with metal studs and concrete floor [condo] covered carpet. What is the best size brad or finish nail to use. Will screw the scarf joints into the studs. Thanks in advance.

If you have metal studs you are not going to have anything to nail into unless a wooden sole plate was used.
Anyway, nails should be 6d (2") or 8d (2 1/2").

Harold is right as rain, munny. You don't nail to steel studs. I will add that even if they used a pt(wood) shoe, you're not going to want to use that exclusively to mount your board, as the baseboard will likely roll out at the top.

I would suggest a combination of PowerGrab(the world's best adhesive for this type of thing) and counter sunk small head self tapping screws.

Use the adhesive on any boards that will sit flush to the wall. Until it's kicked, if you can, you'll want to use something to push the board against the wall. If you don't have another wall close by, simply drag up a chest, trunk, etc, and use blocks to establish a push. Don't worry, it's initial set is quick, so you won't have to walk around your furniture for an hour.

Personally, I would pre-drill and counter-sink every single mounting point(about every other stud, if the walls are straight) as well as use the adhesive. You may very well not want to do this, since you need to fill and sand every screw hole. Although you might not want to do that, you will probably need to use some screws, since walls often aren't straight and if the wall is cupped, you will need the screw to pull the board into the plane of the wall.

Thanks,
Json

I agree with schwim, what we've used in the past was call finish screws, there a square drive small headed screw that will self sink the head, there usually about 2 1/2" long.
And using the power grab will help a lot.