How much memory a DF can have?

With all CIQ apps starting with CIQ 2.x, the VM takes about 4k of the memory.  So 32k is really 28k available to the app.  It's the same for all app types and all devices with 2.x or greater.

Please have a look at

C:\Users\username\AppData\Roaming\Garmin\ConnectIQ\Devices\fenix6\compiler.json

which is part of your SDK installation.

The first section lists the allowed size for DF which is

"memoryLimit": 32768,
"type": "datafield"

Hmmm, that's an unfortunate decision from Garmin's side, they could've still provide the "advertised" amount of memory, which besides of having more memory would also be an absolute number, not something vague like 32K-{~4k that the VM uses}, and who knows if even that amount can change with future FW updates (I mean if they change something in the FW can it also change the amount of memory available?)

Which is what I also sent from the official docs, but it is not what you actually have (at least in the simulator):

But jim_m_58 explained it. It would've been much better though if Garmin would clearly state these things...

The 4k has been consistent since CIQ 2.0.  What you gain is an app can have far more objects.  Look at view memory on a CIQ 1 device then on a ciq 2 device

A CIQ 1 device:

Same widget on a CIQ 2 device:

The 512 max of objects caused issues on CIQ 1 devices

If you look at the bottom line in the sim, you clearly see the max for your app on a device.