UI:  Change transform value 'scale' to 'size'

There's no reason to represent this value in terms of scale.  Scale is a
useless value for users to use.  What are they going to enter, 0.5?
2.0?  0.25?

Even if it can be subject to change by the source itself, and even if
it's still converted to scale internally, having it display the base
source size value is much more ideal for the user.