A while back I enlarged my brush to 600px and for some reason Krita doesn't forget that I did.

I tried ticking the "Temporarily Save Tweaks to Presets" to no avail.
Looked in all ~/.config/krita* files for the value of 600 and found nothing.

Any ideas?

(*) I'm running Krita v4.1.5 on Ubuntu 18.04 Desktop.

'Temporarily Save Tweaks To Presets' only has an effect during a krita session and will not cause brush size changes to be saved after a restart if the size change is done using the Size slider on the Toolbar.

It may be that within the 'Edit brush settings' window, you altered the 'Diameter' parameter, which would be persistent between sessions. If so, there will be a small yellow triangle icon at the top to indicate that changes have been made to the default settings for that brush preset. Next to that would be a small 'circulating arrows' icon that will restore the brush preset settings to default for you.

If you want to have a particulary large (or otherwise different) brush preset, you can make changes in here and then use Save New Brush Preset, giving it a different name and it will appear in the Brush Presets docker.

The ~/.config/krita* files do not contain resource (brush presets, etc) settings. These are in the extensive ~/.local/share/krita/ folders but it's better to use the various krita in-app settings and editor facilities to make changes. (The brush preset files are not human-readable anyway.) If you want to explore and experiment with manually editing these files (which can NOT be recommended), you should make a backup of that entire krita folder (and also the ~/.config/krita* files as a precaution anyway) so that you can restore things after the inevitable mistakes you would make.

The latest version of krita is 4.1.7 if you want to be up to date.