I've noticed that with a char height of 24 px the Asian glyphs are sometimes hard to identify, while the normal letters are way too big.
Maybe another option would be nice: "adjust size to visual detail" - though that would need some mathematical calculations based on the pixels.
Another solution might be a widget for the unicode blocks, where you define a scaling value (e.g. the chinese symbols with factor = 2.0 -> 200%).
So, next to the unicode blocks there is already this checkbox where you decide whether to include this block. Next to it could be a number field with default value of 1.0 (100%) and the user would be able to change that number.
Both solutions would take the Size (px) (Line Height) as a suggestion for the biggest glyphs in the output image:
32 px Size means: chinese symbols with 32 px (which is 200%), and normal letters with 16 px (which is 100%).
Your tool is very cool!