How to make an enlargement system that saves?

You need to use something called Datastores. These are very complex. Here is how you would save: (put in Script in ServerScriptService)

local DSS = game:GetService("DataStoreService")
 local SizeStore = DSS:GetDataStore("SizeSave")
------------------------------------------------------- Start of part 1
game.Players.PlayerAdded:connect(function(player) -- when a player enters
	local leaderstats = Instance.new("IntValue")
	leaderstats.Name = "leaderstats"
	leaderstats.Parent = player
	local Size = Instance.new("IntValue")
	local Size.Name = "Size"
	local Size.Parent = leaderstats
-------------------------------------------------- Start of part 2
	local good, bad = pcall(function()
		local data =SizeStore:getAsync(player.UserId .. "size")
	end) 
	if good then
		Size.Value = data
	else
		warn("Error:" .. bad)
	end
----------------------------------------------------- Start of part 3

game.Players.PlayerRemoving:connect(function(player) -- as a player leaves
	local leaderstats = player:FindFirstChild("leaderstats")
	local good, bad = pcall(function()
		SizeStore:setAsync(player.UserId .. "size", leaderstats.Size.Value)
	end) 
	if good then 
		wait()
	else
		warn("Error:" .. bad)	
	end
end)

This may seem complex. I divided the script into 3 parts using ---------. The 2 lines before part1 creates what is called a "Datastore." It is basically a table that saves between servers of a single game (for a more complex answer, visit the wiki or devforum). The first part creates a leaderboard when the player enters. The second part fetches data from a datastore and links it to the leaderboard. The third part activates when a player leaves, and will save the data from the leaderboard to the server. NOTE: This will not work in studio. TEST IN GAME.