Whenever questions arouse, the wiki is always the first place to check. Here is a link to the wiki page on a regular, normal FOF:
http://wiki.srb2.org/wiki/Linedef_type_100
Under the part where it says setup, it should give you an idea of how to make it. But, for the hell of it I will also tell you.
1.) Tag the sector that you want to have a platform above by right clicking the sector and giving it a number tag.
2.) Create a sector, preferably outside your general map area (the sector can be any size or shape, but it is usually best to make it a square or triangle, and you will understand why in due time).
3.) Right click on a line on the new sector, and give it the linedef 100 (for a normal FOF), and make it's tag for the linedef the same tag as the one from the sector which you wish to have a platform above.
4.) This part is the part of simple resizement. Simply change the floor and ceiling heights of the new sector to affect the height of the now made FOF.