How should I define the size and distance of my targets?

Hello,

When creating a imageTarget, it allows me to add the Width and height. However I don't know how to exactly calculate the size in Unity units.

I have a print pieces that is 8.5x11 that I'm using as a imageTarget.  What size should I use to equal units in Unity? Could I use the equivalent  of 8.5 width in pixels? And, does the height matter?

Also, can I control the distance of when the imageTarget tracking is found to activate the imageTarget? Basically, could I define a certain distance of when the imageTarget is activated?

Lastly, could I target a real world billboard from the ground as an imageTarget?

Thanks

D