If they are in in .avi format you are going to have to change them over to something the ps2 can read. Some programs will do this automaticlly, such as Nero Vision Express, which is what I usually use. If you can't get Nero Vision Express, look up a free program called VSO Divx to DVD. It is a small simple program. Use it to convert the files over to a dvd format. Then of course you will have to use what ever dvd burning software you have to burn those files to a dvd.
One thing I strongly suggest is making sure that you have good blank dvd's. The cheap stuff will not work well at all, especially in ps2's and xboxes. I have an xbox and at first tried using staples brand and they hardly worked at all. I use Verbatim brand now and I cannot recommend them enough. My xbox reads them just fine, and of course regular dvd players do too.
I know VSO Divx to DVD has setting where you can change the quality. You can try lowering the quality to fit more on, but obviously it is at the expense of quality. Other then that I am not sure. I myself perfer quality, even if I have to use more dvds. Not too long ago I burned a full season of the simpsons and it fit on 2 dvd's, which isn't too bad.