Are these files you download avi's? (DivX,XviD format) If so then they will always be larger when converted to DVD format files. I would just try burning two movies to one disc to see if the results are up to your own standards. Two movies that are one hour each in length MIGHT look okay but I would only do one per disc if they are closer to an hour and a half in length. Also the quality of the original file (How much compression they used to convert it to the AVI file) will come into play also. If it wasn't converted properly then no matter what you do, you won't be able to get a good copy to DVD. Sometimes you just have to experiment and waste a couple of blank DVD's to find out. =]
why waist any disks! get an rewritable disk and experiment.... this way if you need to erase you won't waist anything! I recommend the Sony DVD+RW disk. they code out to RICOHJPN and it's a great rewritable. I have 25 of these but I hardly use any of them.... just in case I have a project.... the project never came up..... but it's nice to know that I have them available to do any job that might come along.
THANKS for these useful replies. Much appreciated! They both seem right on the money.
I also posted this question to another ConvertXtoDVD forum, and the moderator gave this reply, which I'll post here in case it's of any interest. It pretty much confirms what you have said, above:
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There is no definite answer, as you said, it depends on many aspects.
Screen size, AR, packing ratio, amount and quality of audio tracks, amount of subtitles and finally type of movie.
In most cases you can fit two to one DVD5.
Only real way to try is to convert first one movie, then check file size of converted folder. Then convert second one and check size.
If sum of those two fits to one DVD5, then they fit with no loss of quality.
Of course you have to reconvert those two (or use DVDShrink to combine them with no menu) to get proper menu.