[Fizzlemizz]

To save information between sessions you need to add a setting you your .toc file

for use by ALL characters (or set up a character specific mechanism here):

## SavedVariables: xxx

Information saved only usable by the current character:

## SavedVariablesPerCharacter: xxx

If your .toc had

## SavedVariablesPerCharacter: DarkruneDOTDK_DATA

To save/set the font size you could do something like:

local function SetFontSize()
    local fontFile, _, fontFlags = ChatFrame1:GetFont();
    ChatFrame1:SetFont(fontFile, DarkruneDOTDK_DATA.fontSize, fontFlags);
end
 
local f = CreateFrame("Frame") -- a frame to handle events
f:SetScript("OnEvent", function(self, event, ...) -- Event handler
    DarkruneDOTDK_DATA = DarkruneDOTDK_DATA or { -- check if your SavedVariables table exists
        fontSize = 12,              -- If not, create it and set some defaults
        someOtherThing = 99,
    }
    SetFontSize() -- Then automatically set the saved font size each login
end)
f:RegisterEvent("PLAYER_LOGIN") -- Event to listen for. This event fires when you login after all addons have loaded
 
 
function DLU:SlashCmdHandler(msg)
    local command, arg2, arg3 = strsplit(" ", msg);
    command = command:lower();
    if command == "size" then -- We want to set a new font size
        DarkruneDOTDK_DATA.fontSize = tonumber(arg2) -- New size, save it to your SavedVariables table
        SetFontSize() -- Update the chat frame with the new size
    end
end
 
SLASH_DLU1 = "/dlu";
SlashCmdList.DLU = function(commandName)
    DLU:SlashCmdHandler(commandName);
end

SavedVariables are global so the name you use for yours should be unique to your addon ie. something like DarkruneDOTDK_DATA