How check used flash size ESP32

The last few days I’m struggling with fonts. It proves to be really very hard to get a decent readable output on a TFT-screen. As stated in this post: Can't use FreeFont FreeSansBold18pt7b I manage to get a 9 pt font and a 12 pt working, but not this two fonts in the same sketch.
Both fonts are under 1600 bytes and as soon as I load a font size ~5000 bytes, the screen is just blinking very fast; I suspect the ESP32 is rapidly rebooting, can’t check that because there’s no serial output.
Any suggestion is welcome; the project’s state/versions is/are in GitHub, see the mentioned post.