SourceRectAtTime doesn't work with variable fonts

Try adding this expression to a text layer above the text layer you want to animate:

testLyr = thisComp.layer(index + 1)
FontSize = testLyr.text.sourceText.style.fontSize; 
SRaTHeight = testLyr.sourceRectAtTime().height;
SRaTWidth = testLyr.sourceRectAtTime().width;

ratio = SRaTHeight/FontSize*100;
"Font Size = " + FontSize + "\r" + "Height = " + SRaTHeight.toFixed(1)  + "\r" +  "Width = " + SRaTWidth.toFixed(1) + "\r" + ratio.toFixed(1) + "%"

As long as you use text animators instead of scale, this will give you just about everything you need.

You can also add modifiers to the value by incorporating scale into the expression.

Hope this helps.