I have here quite a lot DVD's which I want to convert them to H264 video files, with of course good quality and size friendly. I know how I can encode and I know the process of converting DVD to MKV/MP4, but some things are still unclear to me especialle the size-aspect ratio. HD encodings are easy to execute, just convert the 1080I/P to 1280x720P with a de-interlace filter and done. For 16:9 SD content things are quite difficult because there is no online standard for 16:9. Some people are encoding 16:9 in 704x396, another encode it in 848x480, or 720x405. But what is a good size for 16:9 encodings? I have some DVD with progressive source and DVD's with interlaced source, but what is a good size to convert to?

There is no "best" on these forums (per the rules), and there certainly isn't an ideal resolution. The original resolution is 720x480 anamorphic to a display size of 853x480 (or ~875x480 if you're an ITU resize fan). And whose "online standard" are you talking about? Plenty of downloadable content is available as both 720p and 1080p. Thirdly, 1080h->720h is not "deinterlacing". You're confused about vocabulary. I suggest you do a lot more reading around here and then decide on your own preferred resolution. Nobody can tell you what you prefer.

You basically pick what's most important to you: Detail (more size, more resolution), or size (lower bitrate, lower resolution), you'll have to experiment to find your favorite tradeoff. Mine keeps moving and I'm certain it won't be the same as yours. If you want maximum detail without wasting any bits, just crop and encode it anamorphic. Most x264 guis will do this for you.