im 5'9 145 lbs would a small be a good...
im 5'9 145 lbs would a small be a good size
I'd probably do a medium if you don't mind it being a lil' loose. A small will fit slimmer as its a softshell. If it were outerwear I would say a small for sure.
You could probably run either.
Either, it depends on how you like to run your kit. Medium will be baggy, small will be tighter.