70 kb? haha holy shit, i'd be screwed, the size of my current sig is almost 200 kb haha.
i know that saving a sig as a jpg file instead of png can make it a lot smaller, but on some graphics it actually makes a difference. and some even require to be saved as a png. i tend to like to save all my graphics as png files, in fact i've been saving all of them as pngs for years.
but if there's really a need for a limit, either to resolution or size, i think everyone can live with that.