I know that Converting realplayer files to VCD is to use winavi-video-converter but let's say the rmvb file is about 1hours10minutes and the size is around 535MB but if I convert to VCD will the file size be more than 680MB considered that a CD can only store 80minutes of data?
The size capacity of the CD is different from the 'time' in it. You can have 80mins of video CD with 500mb of size.
In this case, you don't care about the size at all, you just have to care about the time. You can stuff the whole video in it. Remember 80mins=1 hour 20mins?