The 16:9 box for 480p is 854×480. An MKV with the dimensions 1920×800 (2.40:1) will fit that box with the dimensions 854×356 (using a modulus of 2).
If you have, say, an MKV with dimensions 1920×796, it’s not meant to be that way. It’s due to one of two things. Two pixels were accidentally cropped out of the original on two sides, and they’re retaining the aspect ratio by retaining that error; or the wrong modulus was used, and it’s now technically in the wrong aspect ratio.
Either way, when resizing to a 480p MP4, pretend you’re dealing with the usual dimensions. (In this case, assuming 1920×800 to convert to 854×356.) You might be fixing the aspect ratio, or slightly altering it, but it will not be visually discernible.
The image you use should be a high quality JPG or PNG at either 3840×2160, 2560×1440, 1920×1080, 1280×720, 854×480, 640×360, or 426×240.
I know that, after your original is re-encoded by YouTube, audio will be 192 kb/s at 1080 and 720, and drop to 64 kb/s beneath that. I don’t know if higher resolutions result in better bitrates.
Use a standard audio bitrate above 192 kb/s (if that’s what you’re aiming for). I suggest 384 kb/s. With every encode you should start with something larger than the result.
As far as I know, the only program that allows you to use a single image for the video stream is TMPGEnc. You can also use a program like Vegas or Premiere to create and merge audio and video streams.
Programs that I know won’t help are Handbrake, XMedia Recode, Avidemux, and Freemake, because they only accept videos as video input.
1080, 720, and 480 refer to the height of the video. But if you have, say, an MKV that is 1920×800 (2.40:1) then that MKV is considered a 1080. The reason the height of 800 puts this in the same category as a video with dimensions of 1920×1080 is that they both fit into the same box. The box for 1080p is 1920×1080. So when resizing you are always fitting videos into the 16:9 boxes of 1920×1080, 1280×720, or 854×480.
If you’re converting from 1920×800 to a 480p video, it would be incorrect to change the dimensions to 1152×480, as that exceeds the 16:9 box of 854×480. The correct dimension is 854×356. If you’re converting to 720p, then it would be incorrect to change to 1728×720. The correct dimension is 1280×534.
The term “aspect ratio” with DVDs is usually applied to the ratio of width to height of the video. Such as 4:3 or 16:9. But there is also the pixel aspect ratio. YouTube, for instance, will only accept videos with a 1:1 PAR, i.e. “square pixels”. Everything Blu-ray has square pixels. DVDs, however, do not.
DVDs are 720 pixels wide by 480 pixels tall, but depending on its two modes, the width is either shrunk or expanded. (This is for NTSC encoding, as opposed to PAL.) Depending on whether the video is 16:9 or 4:3, in square pixels the real width and height is either 854×480 or 640×480.
So NTSC is North America and Japan and a few other places. It’s just a kind of encoding of the original video. PAL is different regions of the world. A PAL DVD will be 720×576, and its pixels will be shrunk width-wise beyond both 4:3 and 16:9. If it’s set as widescreen (16:9), then to get square pixels you would change the width to 1024×576. If it’s fullscreen (4:3) you would change the width to 768×576.
Either from ripping your own Blu-rays to MKV, or downloading an MKV, you have a very large video file that needs to be standardized by encoding to MP4, which is more accessible for devices. (And smaller in file size.) A Blu-ray player will play a data DVD or data Blu-ray with either MP4s or MKVs on it, and your computer will play both MP4s and MKVs, but you might need to stick with MP4 on other formats, such as with phones or tablets. Most importantly, you need to encode to MP4 to save space.
The best software for Windows (at least my version, which is Win 7) is XMedia Recode, which is freeware. I first get an MKV that’s significantly larger than the resulting MP4 will be. (The MP4s I make — and I recommend this setting — use 2,000 kilobits per second bitrate, which works out to about one gigabyte of size per hour of video. So a two-hour movie will encode down to a two gig MP4, and I would choose an MKV original of at least four gig size.)
The dimensions of the video (such as 1920×1080, 1280×720, and 854×480) don’t matter in relation to the eventual file size of the MP4. Just the bitrate. I think of the bitrate as representing the amount of detail packed into the video. One side effect of this being true is that you need to consider the appropriate bitrate for various dimensions. You could make a 1920×1080 video with a bitrate of 2,000 kbps, but it’s going to look the same — and be the same file size — at 854×480. I think of 2M as being appropriate for 480p, 4M as being appropriate for 720p, and 8M as being appropriate for 1080p. You can do whatever you want, but for a video to be a “true” 1080 — in a relative sense — it should have a bitrate that reflects its dimensions.