i used to have the very same problem...
this is how i figure out the scale so it looks right in FS
the thing is, you have to set gmax for 1 unit = 1 meter and THEN you have to measure and resize your model according to the new figures...
what i do after configuring the scale settings is take a known measure such as wingspan and use the tape tool.
specify the lenght of the tape as the plane's wingspan. you'll see it will not fit on the plane... probably will be much smaller.
then all you have to do is uniform scale your model so that it's wingspan matches the tape.
now it should look normal

hope it helped
Moach