How can I get the area between multiple parts?

I have tried quite a few methods, I am trying to get the area between multiple parts so the item I place down can only be placed n that area, and not on the outside of that area.

Here is an image of the area: pasteboard.co/HVrdQ2a.png

local m = {}

-- Priority Variables
local uis = game:GetService("UserInputService")
local runService = game:GetService("RunService")
local rs = game:GetService("ReplicatedStorage")
local furniture = rs["Furniture"]
local remotes = rs["Remotes"]
local confrimPlacement = remotes["ConfirmPlacement"]

-- Secondary Variables
local studio = workspace:WaitForChild("Harbour Studio")
local studioFurniture = studio["Furniture"]
local owner = studio["Owner"]

--Model Placement Varibles
local plr
local mouse
local itemsFrame
local finished
local node
local preview
local pp
local selection
local canPlace
local rot = 0
local placing



function previewItem()
	if (placing == false) then
		return
	end
	
	local allNodes = studio["Nodes"]:GetChildren()
	for _,n in next, allNodes do
		node = n
	end
		
		if (owner.Value == plr.Name) and (pp.Position - node.Size).magnitude < (node.Position - pp.Size).magnitude then
			selection.Color3 = Color3.fromRGB(0,155,0)
			canPlace = true
		else
			selection.Color3 = Color3.fromRGB(155,0,0)
			canPlace = false
		end
		local angle = CFrame.fromEulerAnglesXYZ(0,math.rad(rot),0)
		local vec = Vector3.new(0,preview.PrimaryPart.Size.Y/2,0)
		local cf = CFrame.new(mouse.Hit.p+vec)*angle
		preview:SetPrimaryPartCFrame(cf)
end



function m.start(p, modelName, frame, fin)
	plr = p
	itemsFrame = frame
	finished = fin
	itemsFrame.Visible = false
	finished.Visible = false
	mouse = plr:GetMouse()
	--node = studio["Node1"]
	
	preview = furniture[modelName]:Clone()
	preview.Parent = workspace
	pp = preview.PrimaryPart
	selection = Instance.new("SelectionBox", workspace)
	selection.LineThickness = 0.05
	selection.Adornee = pp
	mouse.TargetFilter = preview
	for i,p in next, preview:GetChildren() do
		if not (p.Name == "Hitbox") then
			p.Transparency = 0.5
			p.CanCollide = false
		end
	end
	placing = true
	runService.RenderStepped:Connect(previewItem)
end

function m.cancel()
	if (preview) then
		placing = false
		preview:Destroy()
		selection:Destroy()
		plr = nil
		mouse = nil
		node = nil
		mag = nil
		preview = nil
		pp = nil
		selection = nil
		canPlace = nil
		rot = 0
		itemsFrame.Visible = true
		itemsFrame = nil
		finished.Visible = true
		finished = nil
	end
end

function m.rotate()
	if (preview) then
		rot = rot +1
		preview:SetPrimaryPartCFrame(preview.PrimaryPart.CFrame*CFrame.fromEulerAnglesXYZ(0,math.rad(rot),0))
	end
end


function m.place()
	if (preview) and (placing == true) and (canPlace == true) then
		placing = false
		confrimPlacement:FireServer(preview.Name, preview:GetPrimaryPartCFrame())
	end
end

return m

I think you mean magnitude. You can use magnitude to find the distance between two objects. Here's an example:

local Part1 = workspace.Part1
local Part2 = workspace.Part2

local magnitude = (Part.Position - Part2.Position).magnitude

Resources:

Magnitude

Accept and upvote if this helped!

Magnitude is basically the distance between 2 points, in your case it's the distance between your 2 parts. The distance is measured in studs so if you were to do

local Part1 = game.workspace.Part1
local Part2 = game.workspace.Part2

local distance = (Part1.Position - Part2.Position).magnitude -- this finds the distance between the 2 parts

wait(3)
print("distance")

What printing the distance will do is tell you the exact amount of studs between Part1 and Part2. You can do much more with it rather than just printing such as finding the distance between a part and a player's torso, then if the distance is less than or greater than a certain amount of studs, you can do something.