This is an easy item to measure. First, as mentioned above you want either end of saw's travel down the rail not to extend past the gibs that keep the saw base aligned properly on the rail. The deflector mentioned above, once installed consumes the leading 32mm (1-1/4"), or so of the guide rail -- this also prevents the saw base (and gibs) to not be placed too far back. When you do not have enough lead before your material and have to start with a plunge, you are best advised to install the stop. This stop has a notch in its corner where you back the saw into -- the saw should sit tight back against the rear of the notch and the left side of the saw base should fit under the side of the notch such that it prevents the saw from lifting. With the deflector installed and the stop backed up too it, the combination leaves the rear edge of the saw base 74mm (3-7/8") forward from the bare end of the rail. With the deflector removed, the stop can be locked further back with its back edge overhanging the bare rail slightly -- this leaves the rear edge of the saw about 38mm (1-1/2") forward of the end of the rail.
The depth of cut also enters into the equation. The deeper the cut, the more room you'll need at the leading end. But when you reach the far end of the cut, the deeper cut will cross the finish line first. With that taken into account, you can evaluate the excess length of rail needed at the both the beginning and end of the cut. This is not as hard to do as this has been for me to describe and can be accomplished easily by placing the guide rail along the edge (like right after a cut) and lower the saw to the depth (or various depths you usually use) and take a look.
I went through this process some time back, and as I recall with a fairly deep depth of cut (around the 40mm setting) the saw needed about 10-3/4" on the lead end with the deflector installed to clear your material and thus prevent the plunge. No matter how you address the leading end, the base of the TS55 can hang beyond the end of the rail about 2" or so before the forward gib has gone off the end. With the saw at this location, the point of contact where the front of the blade meets the splinter strip is about 1/4" before the end of the rail while the rear of the blade meets the strip some 5-1/2 to 6" back from the front of the saw. So once you have established what is needed at each end and then add the length of cut you require, you will know how short a rail you can use. But keep in mind that if you use a rail that 'just' fits, you will need take care to place the rail not only at the correct location, but with the correct overhangs. Furthermore, use on an MFT, Jumbo MFT or cutting table that is also equipped with a fence will again add to you overall needs.
It is rather late, so if I have made any mistakes, please correct me.