With ASS/SSA subs disabled and using the default font settings I set Font Scale to 2.4 and it makes them about perfect size. Adjust up or down by .1 to find perfect size.
With ASS/SSA subs enabled, I ran the Font Scale at 1.2 and didn't mind the size. Perhaps a little small.
I generally leave ASS/SSA subs disabled however out of habit. In my experience, in order to have consistent sized subs, regardless if they are embedded or external, is to disable ASS/SSA subtitles and set an appropriate Default Font Scale. Previous PMS versions had broke Ass/SSa sub option and I got in the habit of avoiding the issue.
Val Killmore: Potato Chip Assassin
SubJunk53--Java 7u4--Acer Aspire 6930 2CoreDuo @ 2.00 GHz 3.00 GB RAM serving to : PS3 Slim -- 2 Xbox360