Yeah, I see that - it looks like Gumby always assumes he is in the middle of the object.

@Helvetosaur so, that works out - but the typed the number is treated as a distance constraint - it moves the scale handle to that dimension. In this case the scale handle starts out as 5 units long and ends up as 12 units long. So the typed dimension is like Scale1D from that corner snapping to 5, then 12 units on the grid.
Admittedly this is somewhat confusing if you expect the number to be an object dimension… BoxEdit style.