Anyone knows the size of the mud flap screws for the GS?

just about everything on this bike has 2 screws on it, if possible take out the second and bring it in to a local hardware shop 

#5061

Size is: Small
Length: Long enough to fit

That's it. It is a totally non-critical part. Whatever fits will work. Of all the fenders with mud flaps I have had and seen, there has been just about everything used. While there IS a "By the Book" correct part, it doesn't mater unless you want to be "100% OEM restored back to original".

you forgot
Threadcount: appropriate