How would I go abut making a script to make players bigger as soon as they spawn in?

I am making a place for a group that I'm in, but the scale is a bit too big. I want to make a script that will resize players as soon as they spawn in.

If you are using R15, you could try something like this:

--Set these variables:
local depthScale = 1
local heightScale = 1
local widthScale = 1
local headScale = 1

game.Players.PlayerAdded:Connect(function(plr)
	plr.CharacterAdded:Connect(function(char)
		local hum = char:WaitForChild("Humanoid")

		local bd = Instance.new("NumberValue")
		bd.Value = depthScale
		bd.Name = "BodyDepthScale"
		bd.Parent = hum

		local bh = Instance.new("NumberValue")
		bh.Value = heightScale
		bh.Name = "BodyHeightScale"
		bh.Parent = hum

		local bw = Instance.new("NumberValue")
		bw.Value = widthScale
		bw.Name = "BodyWidthScale"
		bw.Parent = hum

		local hs = Instance.new("NumberValue")
		hs.Value = headScale
		hs.Name = "HeadScale"
		hs.Parent = hum
	end)
end)

You can look into how these work here.