There is a keyboard shortcut to reload the config. I don't remember what the default is. On the kitty website, go to the page about kitty.conf and ctrl-f reload or refresh.
To check that the config is really reloading, make an edit to the section about theme, tab style, etc.
There is a way to output the actual config as used so you can look for your settings.
If you call kitty from the command line you can optionally use an argument to specify a config file. Iirc it is --config but verify in docs.
What is the location of the config file you are editing?
Post the section of the config file that isn't working?
Try moving the section you are editing to the very bottom of the file to ensure it isn't getting over rided later on.
Ctrl + Shift + - should decrease the font size of the current terminal, which isn't what you are looking for, but could be useful while you sort it out. Is it possible that the font in the config doesn't exist, causing this issue?
I edited my config, and it changed the font size next time I opened kitty. I'm not sure what is happening to you, other than maybe having the config file not where it's expected.
Can you try pressing ctrl+shift+f2 from within Kitty and then adding font_size 20.0? That will make sure you are editing the same config file Kitty is using.
If that doesn't work, I'd try deleting the config and then try again. Kitty should automatically create a new config.