Please note that the database does not contain any previews of images. So the occupied space of library.db might be not that much*. lighttable previews in different sizes are stored in a harddisk cache. This cache seems to be on Windows in C:\Users\[Username]\AppData\Local\Microsoft\Windows\INetCache\darktable\mipmaps-*. This needs much more space if you have a lot of images.
There is a second database data.db, which stores presets, tags and styles. If you plan to work on different systems you maybe want to have same presets, tags and styles there as well.
*check this on my Debian/Linux home:
library.db contains 16k images and has a size of ~79 M.
mipmaps-* folder has a size of 871 M whereby most folders only contain 10k preview images. I don’t want to run darktable-generate-cache to update all previews as it would may take quite some time.