How to increase the size of a part when a variable reaches a certain number?

Hello scripters that are better than me, I am trying to make a script where if the temperature value reaches a certain amount, coresize value increases until it reaches 20. The coresize value should increase the size of a part. Here is what I tried:

temperature = 200
coresize = 4
if true then
	wait(3)
	temperature = temperature +5
end

if temperature >= 210 then
	coresize = coresize + 1
	script.Parent.Size.X = coresize
	script.Parent.Size.Y = coresize
	script.Parent.Size.Z = coresize
repeat until coresize == 20
end

Any help is much appreciated =)

Though I understand your question, your attempt at solving it very much confused me, so I will show you an example of how I would do it and explain as I go. The code below assumes that the script is within a part inside the Workspace.

--We create our values as instances, unlike your variables above

local temperature = Instance.new("IntValue")
temperature.Parent = script.Parent
temperature.Value = 200
temperature.Name = "temperature"
local coresize = Instance.new("IntValue")
coresize.Parent = script.Parent
coresize.Value = 4
coresize.Name = "coresize"
local TempBeforeChange = 201 --The value that will trigger the event

temperature.Changed:Connect(function(prop) --Fires when the value changes
	if temperature.Value >= TempBeforeChange then
		for i = coresize.Value,20 do --Loop from coresize to 20
			wait()
			coresize.Value = i
			script.Parent.Size = Vector3.new(coresize.Value,coresize.Value,coresize.Value)
		end
	end
end)

You can change this at your own desire, as it was just an example!