Burn the converted mkv to avi into vcd.

I have converted successfully my mkv file to avi. The original mkv resolution is 640 x 480.. after conversion to avi the resolution is still the same but the subtitle became smaller though readable on my pc using WINAMP. But when I tried to burn the avi file to vcd using Nero7, the subtitle became more tiny and not readable if I am 1 foot away from the TV and aside from that the resolution has changed to 352 x 240. What will I do to maintain the size of the subtitle from the original file?