Roger, If the bar you have is a stock bar from a 97 GT I think it's a 16mm bar, which is what I took off my 98 GT Wagon. To be certain slip open end metric wrenches of various sizes onto the bar til you get a tight fit, and that's the size.
As far as fitting to an L, did you notice any mounting points for the end-links on your L? You may in fact need some additional hardware, besides the end links to make it work.
This site has some good pics of Impreza application but you can see the way the links mount to the rear suspension. AZScooby in the links section of this Board.