Text in video is a bit of a trick.
There are two things to bear in mind.
1. you are working across two different pixel sizes – square pixels on the computer, oblong pixels on tv.
If you create titles in a graphics package you need to create them in one size and then squeeze them. The measurements depend on your tv system (ntsc, pal etc.) for uk SD Pal you create in 788 and then squeeze it to 768 (x576).
2. Because of the two different pixel shapes what you see on a computer and what you see on a TV are two different things. Ensure you view your rendered version on a tv and you may find that the text is actually fine. Again this is because of the different pixel aspect ratios.
If you search the forum this question has come up a number of times.
Good luck