YouTube converts all uploads to FLV format, even if the original file is already encoded in this format. When you upload a high-resolution video, YouTube gives viewers a selection of lower resolutions to accommodate slower connections. Picture size corresponds to file size in that a higher bitrate is necessary to stream a larger number of pixels, but videos with little movement don't benefit from a high frame rate or require a fast bitrate to stream many largely identical frames. YouTube recommends creating the highest-quality video you can upload as technology continues to improve streaming quality.

Large FLV Files

You can create FLV files with video-encoding software or by exporting Flash video from an editing application, such as Adobe After Effects or Final Cut Pro. When encoding or exporting a video, the settings panel offers options that affect the file's size, including bitrate, frame rate, number of keyframes and resolution. While you can't increase the resolution beyond the source value, you can adjust the other variables for optimal performance. YouTube recommends using the source frame rate and the highest keyframe and bitrate settings. High-resolution videos can display fine detail, such as text and graphics, but may not require the highest keyframe or bitrate settings if they don't contain any fast movement.

Medium FLV Files

When you choose a medium resolution, such as 480p, YouTube gives viewers a choice of lower resolutions, such as 360p and 240p. For standard-quality 480p video, YouTube recommends a 2.5 Mbps bitrate, as marginally higher bitrates don't improve streaming quality and only increase file size and upload time. For high-quality 480p video, YouTube recommends a 15 Mbps bitrate, resulting in a file size larger than standard-quality high-resolution video. You need to upload video only at this bitrate to showcase a professional video production studio or movie project. While high-speed Internet connections are increasingly common, medium-sized FLV files upload and stream quickly, displaying enough detail in videos that don't require viewers to read text.

Small FLV Files

Low-resolution videos can look extremely pixelated in full-screen mode but stream fastest and may be detailed enough for many applications. If the most important part of your video is the audio track, you can keep your file small without compromising your viewers' experience. YouTube recommends a bitrate of 1 Mbps for standard-quality and 5 Mbps for high-quality 360p video. While a 360p video is the lowest-resolution video you can upload, YouTube offers viewers a 240p option when streaming.

Video Encoding

While YouTube supports uploading videos in FLV format, it's optimized for MP4 files containing H.264 video. If you convert or export video before uploading it to YouTube, choose the “Fast Start” option from the settings menu to place the “moov atom,” or meta data, at the beginning of the file. The YouTube video player displays video in a 16:9 aspect ratio and adds letterbox or pillar bars to videos as necessary to fit the screen. If you try to change a video's aspect ratio from 16:9 to 4:3 by adding letterbox bars, YouTube changes it back to 16:9 by adding pillar bars, reducing the video's actual resolution.