Before installing GhostBSD I read this : https://wiki.ghostbsd.org/index.php/Har ... quirements

Minimum System Requirements

64-bit processor
4 GB of RAM
15 GB of free hard drive space
Network card

So I allocated 25GB so that I don't run out of space in future but after just one week of use I see that I have only 11.6GB of free space left.

Is 25GB too low ?

15G is Minimum, not a perfect case scenario, and 25G of space for me is low, but for you, it depends on what you want to do with GhostBSD.
Me it is on SSD 512G, it just depends on your use case.