SUMMARY
My monitor is high DPI with a pixel density of 166ppi and a screen resolution of 3840x2160. I think that kind of screen is getting more and more popular.

Relatively often, I take (partial) screenshots to share some news or similar with friends or collegues via various channels. Now due to the screen resolution, those screenshots always tend to be quite large in terms of data / file size. To reduce that, I either need to save the file, open in gwenview (or similar), resize and then share or, what I'm doing now most of the time, un-maximize the window to an acceptable size, then take a screenshot and then maximize again. I think there could be better options to handle this.

My proposal: Next to the resolution, Spectacle could show also the dpi of the screen, e.g. 166 dpi in my case. The user can optionally set a target dpi, e.g. 100 dpi. That way, the downsize is independent of the actual size of the region. For bonus, it could estimate what the reduction in file size would be, but I don't think that's really necessary.

BTW, at first I thought that an easy solution would be to move a window to my secondary screen, which only has a resolution of 1920x1200. But to match the two screens, that screens effective resolution is actually upscaled, meaning that for screenshots, it doesn't make a difference (or at least that's what Spectacle tells me).