Basically possible but tricky in detail: I tried adding the size information with method 1 described on https://nsis.sourceforge.io/Add_uninstall_information_to_Add/Remove_Programs....
This works well enough for Geany itself but provides wrong information for G-P: the mentioned GetSize function works by determining the used disk space of the installation directory *after* installation. This fine for Geany and gives accurate numbers. However since G-P has to be installed in the same installation directory as Geany itself, the G-P installer would include the existing Geany installation into the calculation and so reports wrong numbers.
I think telling the user wrong numbers is worse than not telling anything.
In theory, the best solution probably to sum up the required disk space numbers in the installer's Components page and use the result for the uninstall information. But I didn't find a way to get these numbers in the NSI script.
If anyone has an idea and/or other solution, just re-open.