This annoyed me enough that I made a handy cheat sheet for myself, both in picture form, and as a table. They are posted below, and all the information comes directly from the preferences for Video DownloadHelper (right click on icon -> "preferences" -> "capture" tab on top -> "YouTube" tab below "capture" tab).

Type | Format | Video | Audio | Name |
---|---|---|---|---|
[HQ18] | MP4 | 360x270 / 480x360 | Stereo | Medium |
[HQ38] | MP4 | 4096x3072 | Stereo | 4K |
[HQ37] | MP4 | 1920x1080 | Stereo | 1080p |
[HQ22] | MP4 | 1280x720 | Stereo | 720p |
[HQ35] | FLV | 854x480 | Stereo | 480p |
[HQ34] | FLV | 640x360 | Stereo | 360p |
[HQ6] | FLV | 360x270 | Mono | HQ6 |
[HQ5] | FLV | 320x240 | Mono | HQ5 |
[HQ17] | 3GP | 176x144 | Stereo | Mobile |
[HQ13] | 3GP | 176x144 | Mono | HQ13 |
[HQ82] | MP4 | 480x360 | Stereo | HQ82 |
[HQ83] | MP4 | 360x240 | Stereo | HQ83 |
[HQ84] | MP4 | 960x720 | Stereo | HQ84 |
[HQ85] | MP4 | 720x540 | Stereo | HQ85 |
[HQ36] | 3GP | 360x240 | Stereo | HQ36 |
[HQ37] | MP4 | 1920x1080 | Stereo | 1080p |
[HQ38] | MP4 | 4096x3072 | Stereo | 4K |
[HQ43] | WEBM | 640x360 | Stereo | WebM360p |
[HQ44] | WEBM | 854x480 | Stereo | WebM480p |
[HQ45] | WEBM | 1280x720 | Stereo | WebM720p |
[HQ46] | WEBM | 1440x1080 | Stereo | WebM1080p |
Another way to make the plugin a little easier to use it change the "Display HQ prefix" setting. Here is what a sample Youtube download menu looks like before changing it.

Next, I change the "Display HQ prefix" setting to "[Np WxH]". This tells the plugin to display the name of the type and then the dimensions.

Finally, here is what it looks like now, after changing the setting:

Hope this post helped someone! If you have information to add that could help others, feel free to leave it below in the comments!