How to size an object on one side only?

local player = game.Players.LocalPlayer
local TweenService = game:GetService("TweenService")
local mouse = player:GetMouse()
local increment = -0.5
local debounce = false
mouse.Button1Down:Connect(function()
	print(mouse.Target.Name)
	if not debounce then debounce = true
		if mouse.Target and mouse.Target.Name == "Dirt" then
			local goal = {}
			goal.Size = mouse.Target.Size + Vector3.new(0,increment,0)
			local tweenInfo = TweenInfo.new(1)
			mouse.Target.CFrame = mouse.Target.CFrame + Vector3.new(0,increment/2,0)
			local tween = TweenService:Create(mouse.Target,tweenInfo,goal)
			tween:Play()
			print(mouse.Target.Size.Y)
			wait(0.5)
			debounce = false
		end
	end
end)

I've tried doing that, but sometimes it glitches out and makes the parts hovering off the ground... is there a better way to resize objects? I've also tried using the :Resize() function, but it only takes int values.