I am looking for a low resulution data set (grid) of the world that contains the elevation above or depth below sea level, respectively. For my purposes, it's enough if I can display the entire data set on a laptop screen. For example, with a 20 by 20 minute grid, that would be 1080x270 pixels. It could by slightly larger, though.
You can use the ETOPO dataset by NOAA NCEI. It has a 1' resolution, which you can of course downsample according to your needs.
Several derived data sets are available as well.