[Stiglr]

Size uses units (whether meters, feet, inches, whatever you want them to be) as the size determinant.

So, if you fill in the X Y and Z boxes of "size to" to 3, 1 and 5.03 respectively, and click the Size to button, you will have an object that is three "units" wide, 1 unit high, and 5.03 units long. If you turn the grid on in your viewpoint you can verify this.

Scale to changes the % of the original length in all three axes. If you fill in the X Y and Z boxes of "scale to" with 90, 90 and 100, respectively, then click the Scale button, you will make the object 10% thinner side to side, 10 percent shorter height wise, and it will retain the same length. You can also enter values in the boxes greater than 100% if you wish to enlarge.

Also note you can repeatedly hit the Scale button and get the same results, whereas with Size to, it's "absolute" so you can only press it once.

I often use the Scale function to fit items that I create by tracing a diagram (which indicates no size), and then place them into another object (as in, creating gear for an aircraft cockpit, where I have a photo or diagram of the item in place, but the angle is skewed... and I have a separate photo or diagram of the part shot at a 'straight on angle'. I use the straight on art to trace a polygon and give it the correct thickness. Then, I set the scale at 90%/90%/90% if I want to get it smaller, or 110% across the board if I want it larger. Then I place it in situ, in the aircraft fuselage, and click the scale button until it occupies the correct relative space.