Converting mkv in Format Factory

Hi,
I have a new video in MKV format and I tried to play it in my DVD player. It didn't play. This is not the problem btw. i know my DVD player will definitely play an avi format video, so I tried to convert the MKV file. When I played the converted file, the subtitles became so big that it fills the whole screen, but in the MKV format the subtitle is normal. How can I retain the size of the subtitles?
Thanks.
