What drawing program are you using? That's the first question I'm gonna ask here. Some programs offer a lot more versatility with image/resolution size than others.
Second, what sizes are you using to draw at and what resolution are you using? Just letting you know from my own personal experience, don't be using the default 72 dpi for your standard resolution, it's crap. Use 300 or higher, you'll get much better results.
Remember that there's no vertical limit to your pages (so long as the total file is less than 2MB), they just need to be no wider than 940 px.
So what I'd suggest is to draw it at full size (whatever full size is for you) and full res, and then just shrink the image down to 940 px width for when you save it as JPG (although I suggest not saving over the original file size, there's no harm in keeping back-up files at the original size and resolution!)