If you try to build multi-tile signalbox in place where only one free tile is, it is built and overlaps some random building, or even the rail track itself. To make it worse, after deleting the signalbox, the overlapping half is still in place and cannot be destroyed. See the save https://uran.webstep.net/~vladki/simutrans/trnava.sve
Find trnava station, and the space between the station building and bus terminals. Some more tests also at trnava junction station. Those did not overlap the track and could be later deleted.
I think the signalbox shoud not build in such case and give a warning. Also it should show how many tiles it needs, just like when you build factories, or station larger extensions.
Last thing I was surprised that I can build signalbox anywhere, not just near the treck. Is that intentional?
The latter is probably intentional; sometimes for location reasons it is not feasible to have it right adjacent. Probably be too much of a bother to be worth limiting the placement further than the natural limit applied by the radius; it's useful for when the track is in a tunnel, too.
It could also be usefull to choose rotation with ctrl, just like for extensions.