Chris,
I am by NO means an expert at Railmodeller, but I have played around with it for several months trying to come up with plans for my layout. Once you learn how to navigate the menus, it's a fairly good layout program. Unfortunately, you can't alter the lengths of rigid track lengths, both straight and curved (That I know of). You can however take a piece of flex track (whatever brand), and go into the "edit flextrack properties" under the element pull down menu, and change the length. There are three sub menu's: Parameter ( can't edit anything), Bend to curve (both radius, and arc angle), and Bend to straight (only length)! You can then put it in the track section and bend it as need to fit.
Hope this helps!
Dave