The .sii files are plain text files. All you need to open one is Notepad, which is included with every version of Windows you will find in use these days. The problem is that .sii files are not associated with it. Right click on an .sii file and you should see a button marked "Opens With". Click on it and set it to notepad.exe.
Making a backup of any file you want to edit is always a good idea, even when you absolutely know what you're doing, because Windows can have a mind of it's own at times, and does not always do what is supposed to do...
Having said that, this is one section from the garage_door.sii file for one door:
{
model: "/prefab/garage/door_expensive.pmd"
animation: "/prefab/garage/door_exp.pma"
collision: "/prefab/garage/door_expensive.pmc"
trigger_distance: 18
sound_path: "/sound/gate.ogg"
}
If you want that sweet spot to be bigger, change trigger_distance: 18 to a higher number, if you want it smaller, use a lower number. Just remember, make a small change and test it, make a small change and test it, etc., until it's how you like it.