Joined 2x1400 lengths gives you about 4" overhang each end. I suppose dependent on the depth you're cutting which for the most part will be 18mm, and given where the center of the blade sits, or the effective cutting point depth for 18mm just back from it pretty much at the back edge of the blade, you're going to have the full base sitting on the rail. It isnt like its a circular saw where you need to place the front of the blade into the end of the board.With 2020 hindsight, 1400 rails are a pain. They are a little short for crosscutting an 8x4 sheet. You really want a little less than a saws length of overhang at each end for a clean run on and off the sheet without the saw wobbling. 1900 rail is dearer but I wish I'd bought that rather than the 1400.
So I would have thought a 2.8m rail on a 2.4m board is ample. Same applies for the width. its 1.4m against 1.2m.
Im pretty sure rails lengths were designed purely on cutting 8x4 sheets which is why each length is 1.4m
If youre really stuck though, currently theres a continuous 3m festool rail for sale on ebay for 200 quid.