The image compression in C3 is supposed to be very efficient (according to Ashley's article below) so if you use a full-scale sprite for development you may not necessarily be wasting space once it's compressed for distribution. You could make test projects with small sprites in one and full-size sprites in another and see what the difference is after export.
This is from section 3, "IMAGE RECOMPRESSION"
"It's also able to losslessly palette-reduce 32-bit PNGs to 8-bit PNGs when the image has 256 colors or fewer, resulting in a much smaller file size, which is particularly effective for art styles which don't use many colors, like retro style pixel art." (emphasis added)
construct.net/en/tutorials/construct-3s-export-optimisations-4