subtitle size problem

[sirun]

good evening,
i have a problem which bothers me very much,
it's quite a long time i use mediacoder, everything seemed very easy for me.
lately i started to convert mkv files to avi...
sometimes everything is satisfactory but other times it doesn't work as i want.
the problem is with subtitles, when i watch the mkv file with vlc, it's ok : the characters are in normal size ; but when converted in avi they are too small, in other words : unreadable on tv...
i tried with a smaller picture resolution ; that didn't change anything
is there something i can do about it ?

thanks