You scale in a modeling package.
...then export as an FBX again.
If your track is only a little shorter in TrackEd than it is in real life and has hills in it - that's normal. TrackEd gives a 2d length - it doesn't take elevations into account - so will pretty much always show a shorter length. The more hills - the more it'll read short.
If the track's showing as tiny or huge and you've had it through 3D Studio MAX during conversion - MAX's unit settings probably still need changed to Metres. The units are in Imperial when you install and there are two places you need to change them:
Customize>Units Setup>Display Unit Scale
and
Customize>Units Setup>System Unit Setup>System Unit Scale
...then any models you import into it in the future will remain the correct size.