11-07-2021, 02:23 PM
The answer is: a big as you can 
In practice, when Gimp requires more RAM than what the tile cache is set to, it swaps to disk and this is much slower (even with an SSD). But If you make is too big, when Gimp starts using lots of memory all other apps on the system will start swapping, and this will slow them down. So, see how much free RAM you have left when your usual apps are up (mail+browser+whatever) and this is a good starting point for your tile cache size.
In practice, when Gimp requires more RAM than what the tile cache is set to, it swaps to disk and this is much slower (even with an SSD). But If you make is too big, when Gimp starts using lots of memory all other apps on the system will start swapping, and this will slow them down. So, see how much free RAM you have left when your usual apps are up (mail+browser+whatever) and this is a good starting point for your tile cache size.