When placing a trigger or track mark, or even a switch lever, go to the triggers and track marks, advanced pullout at the bottom of the panel. You'll see a small window on the left side with a number in it. This number is in meters and by default is set to 20. You can type in any value you want to reduce or increase the radius of the affected area.
As I said switch levers too. Place a switch lever, change to the triggers and track marks tab, go to advanced and you can then reduce the locking radius. This is helpful in some places such as busy yards where you don't want to stop 20 meters from the junction in order to switch the lever. Be careful because setting the radius too small can have some odd effects, and can really look funny too if the train is on top of the junction when switching the junction lever.
To add to this, as I was interrupted by a phone call, to get and apply values, use the corresponding tools on the advanced panel.
