I just got a used 2003 Giant XTC frame which I plan to build into a singlespeed. What BB size do I need for this?
Measure the width of the BB shell, probably 68 or 73. Then you will need to decide the correct spindle length. Spindle may depend some on the crank you use, but more on which positon you want to run the chainring in. I like the looks better when the ring is run in the outer postion on the spider but, if you run a bash gaurd or have chainline issues you may run it on the inside of the spider. I've built several SS bikes in the last year and I think all were either 68x110 or 68x113.
The rear hub can affect the placement of the chainring up front as well, with a cassette type hub you have a much more adjustable chainline than with a screw on freewheel in the rear. Some additional adjustment can be made to the bottom bracket by using spacers there too if you need. It's really not hard, but it kind of has to be figured out. I don't think anyone will be able to give you the answer unless they know the bike and know what kind of hub you are using.