LeaPlusLC.grid = CreateFrame('FRAME') 
LeaPlusLC.grid:Hide()
LeaPlusLC.grid:SetAllPoints(UIParent)
-- Get the REAL Width (w) and Height (h), by real I mean your actual resolution (ie 1920x1080) because WoW works in a way where it has a fixed Height of 768 and then uses the scale to get to the real resolution. Plus, people change the UI scale and that messes things up too
-- For your specific case, since you only want the ratio, it's still correct if you don't do it like this with GetEffectiveScale, although i'm not sure in the case people change the UI scale
-- Also, by using this the grid scales with your resolution and with your UI scale, which was the intended purpose, that's why I didn't even bother having different size grids and instead used a fixed number for the square size that is relative to the resolution (you will see this code in a moment)
local w, h = GetScreenWidth() * UIParent:GetEffectiveScale(), GetScreenHeight() * UIParent:GetEffectiveScale()
-- Calculate the resolution ratio :)
local ratio = w / h
-- This was me choosing the size of grid squares I wanted. This way I know I always have squares that are 1/30th of the true resolution width and later I use this to make sure they're perfect squares and calculate how many I need vertically depending on the aspect ratio
-- Even though this already makes the grid scale like I talked before, you can make it a variable instead so you can use your "small/big/etc" commands with it and have it be even better
local sqsize = w / 30
-- Calculate the number of vertical (width) lines
local wline = floor(sqsize - (sqsize % 2))
-- Calculate the number of horizontal (height) lines
local hline = floor(sqsize / ratio - ((sqsize / ratio) % 2))
-- Draw the vertical lines
for i = 0, wline do
-- To be honest I did this with a CreateTexture because I didn't even know there was a CreateLine like you used so I would probably just keep your CreateLine
	local t = LeaPlusLC.grid:CreateTexture(nil, 'BACKGROUND')
-- This is just to get a different color on the middle line and all others
	if i == wline / 2 then t:SetColorTexture(1, 0, 0, 0.5) else t:SetColorTexture(0, 0, 0, 0.5) end
	t:SetPoint('TOPLEFT', LeaPlusLC.grid, 'TOPLEFT', i * w / wline - 1, 0)
	t:SetPoint('BOTTOMRIGHT', LeaPlusLC.grid, 'BOTTOMLEFT', i * w / wline + 1, 0)
end
-- Draw the horizontal lines
for i = 0, hline do
	local t = LeaPlusLC.grid:CreateTexture(nil, 'BACKGROUND')
	if i == hline / 2 then	t:SetColorTexture(1, 0, 0, 0.5) else t:SetColorTexture(0, 0, 0, 0.5) end
	t:SetPoint('TOPLEFT', LeaPlusLC.grid, 'TOPLEFT', 0, -i * h / hline + 1)
	t:SetPoint('BOTTOMRIGHT', LeaPlusLC.grid, 'TOPRIGHT', 0, -i * h / hline - 1)
end