Although the file sizes of an Arduino library's source code is always small enough to not be worth consideration, some library authors have added supplemental files which make the total file system size of the library unexpectedly large.

An example I have seen repeatedly causing problems by exceeding the maximum storage size for a standard Arduino Cloud account upon import is this popular library, currently ~40 MB:
https://github.com/itead/ITEADLIB_Arduino_Nextion

Another example is the poor decision to include a 30 MB animated GIF with the "Arduino_TensorFlowLite" library solely to illustrate the "L" LED on the Nano 33 BLE board blinking.

Excessive library sizes are inconvenient for users, who might have no interest in, nor even awareness of, the supplemental files that make installation and updates of the library take forever. This also causes an burden on the Library Manager infrastructure, which must index, store, and serve every release of every library.

For this reason, it would be helpful to add a rule that checks for library size exceeding some maximum value (e.g., 10 MB). This could be promoted from a warning to an error when in --library-manager=submit or --library-manager=update modes if we decide to prohibit such libraries from inclusion in the Library Manager index.

Such a rule would encourage library maintainers to package and distribute supplemental files separate from the library itself. That practice will likely be in the best interest of users, who often don't even know (or want to know) where the library is on their system. Library maintainers might also find that supplemental files are unnecessarily large (e.g., raw high resolution images where offering such level of detail is of no benefit).