I've drawn myself a couple of overlays to use here on subeta, but when I shrink them to abide by the size guide, they become very pixellated. Sharpening the image after sort of helps, but the quality is still much worse than the working size. I feel like there has to be a better way. Any advice would be greatly appreciated!

This could be caused by a couple of issues.

First: How big is your working size image?

(This is the most common problem I see. Most people I know work at x2 the final size, up to a MAX of x4 final size. If your image is MASSIVE, it will not resize properly because the data is being eaten as it's resized.)

If this is not the problem, I have a few other suggestions, so just let me know!