When it comes to banners in etterna, the majority of people make them one of two different resolutions.
512x160
1024x230
For backgrounds, there's no standard agreed upon size, however the most common sizes people use are 1280x720 and 1920x1080, though any 16:9 background will work well in Etterna. (Some people still use 4:3 backgrounds too).
For CDTitle's, it can vary significantly depending on what you want to do, however a good rule of thumb is to try keep the image 4:3 for clarity. That isn't always the case though, and cdtitles are a lot more flexible than banners or backgrounds, so you can try whatever you want, since etterna scales the cdtitle accordingly. The resolution I use for my cdtitle is 100x100, and that seems to show up flawlessly in-game, but there are others who have cdtitles that are something like 105x24, and that shows up just fine too. Ultimately any resolution works, you can just trail and error to see what looks best in-game!
Animated CDTitle's are quite easy. I'll use both mine and a charter named velaachlorine's cdtitle as examples, since they're the only modern-used animated cdtitles that I've seen, and they both use a different method. Simply put, you can either use a spritesheet .PNG for your cdtitle, or you can use a .avi!
If you're making a spritesheet .PNG, you need to title it a certain way. I will include a screenshot of someone's animated cdtitle so you can see. It is important to note that, the closer to a square your spritesheet full resolution is, the better. Etterna struggles with spritesheets that are super long in either direction, the animation gets super blurry. To make the cdtitle work, it needs to be named column x row. So for example, if the cdtitle is 3 frames of animation, horizontally. You would title it as they have in the following example:
If you're using a .avi, then its a lot simpler, but .avi cdtitles don't always work, so its best to use the previous method if you can! For a .avi, you simply put it as if its a normal cdtitle and it should work, as shown in the image below.
It's important to note that the larger an image is, the more likely it is for the game to downscale it, and if it has to be downscaled then as it gets larger the amount of time the game takes to downsize it in realtime is exponentially longer. This can cause severe stutter on the scroll wheel, as it takes longer to load the banner due to the time taken to downscale it. This applies to Backgrounds as well. The recommended resolution changes depending on what theme you use. For til death, it's 480p, for Rebirth, it's 720p