All backgrounds used to be drawn on paper, the preferred media being watercolours or inks. It was then a simple matter for the camera operator to put a field chart over a layout or actual background to measure camera moves. It is not so easy for today's digital compositor when the artwork exists only as a file, especially when complex camera moves are required. How can animators and layout artists best convey to the people doing the scene compositing what is wanted?
My recommendation is that you adopt the practice of always starting with a drawn layout on paper that is in correct scale with the animation artwork. In other words, produce the rough of the background as though you were going to colour it by hand on paper. Instead of colouring it, the drawing can be scanned in using a flatbed scanner. Once scanned, it can be used as a reference layer in your paint software which you can work over.
Now, you will have a piece of paper that the compositor can pore over and measure if needs be and the animators can refer to, plus a bitmap file that will precisely match the drawn animation. It may well be that the background bitmap file will need to be scaled down, depending on the dpi settings used when it was scanned, but you can be confident that the background and foreground elements can be made to fit accurately. This is of particular importance if some of the animation is matched to the background.
Bitmap File Sizes
The procedure for producing a piece of digital artwork for use as a background would be as follows:
It is easy to alter the final output pixel dimensions of artwork from any graphics program, so the trick is to end up with a file that is as small as you can safely get away with. Massive multi-megabyte picture files can bring even a fast computer to a crawl if it is also having to do things like camera moves, blurs, adding special effects filters and so on. For top end cinema quality animation, large picture files are a necessity, but for standard TV use, there is no reason to waste your resources dealing with bloated files.
Assume you followed my advice and scanned in your 12F drawing at 200 dpi. This would result in a picture file 2400 pixels wide (12 times 200). Let us say a standard TV picture is 800 pixels wide (actually it's 720). This means that your image is 3 times wider in pixel terms than necessary. That is in fact a good thing, because it means we can track in to the artwork without it going soft and blocky. In this instance, we could comfortably track in to a 4F without loss of picture quality. If we wanted to go in tighter, we would have to increase the resolution accordingly.
If you need some advice on choosing graphics file formats, there are some separate notes