Configuring font size in VSFilter (& DVobSub) doesn't work too well for different sized videos (eg. VCD vs a DVD rip.) Font is too big in some and too small in other.
"Double video size if" helps some (specially since it enhances the font details), but problems still exist.
What about adding the option of specifying a favorite resolution for the selected font size (eg. 24pt for 720x400), but then adjusting this size for the current movie size? (eg. if current movie size is 720x280 then font size would be (280x24)/400 ~ 16pt. I guess it shouldn't be too difficult to code ;)