How do you find the size of a unit's collision box? Or, do you know what an SCV's collision box size is?

Yes, very small. And to find collision you use the debug check show unit collision sizes.

@ TassadarZeratul

Return Cargo is bad as it requires two keyboard presses. You'd have to press a number for the hotkey, C for the action, then another number to go back to the original unit. That's 3 actions on the left hand. Compare that to the movement detection system, whereby you press a number for the hotkey, right click for the action, then another number to go back to the original unit. The second is FAR faster as your left hand doesn't even have to move from its position to switch between two numbers.

Sizes for inverted locations can be determined via DatEdit. Open it up. Find your unit. Under the unit tab, click the sub-tab Graphics. There should be a section called Unit Dimensions. Then use this formula, where LEFT is the left of the location, left is the left of the unit dimension, and min is the minimum of the two parameters.
LEFT = 2 * min(left, right) -1
TOP = 2 * min(up, down) -1
RIGHT = 0
BOTTOM = 0
Now center the location on your unit. You're done. Note that some units like the Science Vessel won't have perfect detection for movement as their graphic size isn't a square. This is also far FAR superior to 0x0 locations in terms of speed.

I also mentioned HP for the spellcasting units, which is something simple that should be done much more often than it is. Everyone was wowed by how Spellsword did it and I couldn't help but shake my head sadly and sigh. In Spellsword, I never looked at my main character. Instead, I stared at my spellcasting units and the burning building blocking them to see how freaking long it would take for the spell to cool down. I could then cast spells at the maximum speed possible. Whereas it was effective, is was...pretty freaking boring.