Change the Size of multiple parts at the same time?

I have 4 parts that I want to each 'tween' their sizes every "X" amount of seconds.

This script tries doing so but I know I'm missing something. Instead this only gets one part in a Folder and only resizes that Part, then it moves on to the next Part:

local boundaryFolder = game.Workspace.Bounds

function increaseBoundaryRadius()
	local amount = 5	
	
	for i, v in pairs(boundaryFolder:GetChildren()) do
		if v.Name == "boundPart" then		
			for j = 0, 5, 1 do
				wait(1)
			 --this is where the parts should be increased every 1 second by adding 5 to their sizes
 
				v.Size = v.Size + Vector3.new(0, 0, amount)
			end
		end	

	end
end

while wait(7) do -- this is "X" amount of seconds
	print("Starting")
	increaseBoundaryRadius()
end

The problem here is the way you have nested the loops. You get the individual items then per item you scale them not as a whole set.

This is a simple fix just swap the loops but this would be inefficient as we would get the children each time so it would be best to make a new table of parts we need to scale:-

local boundaryFolder = game.Workspace.Bounds

-- local functions need to be defined before they can be used
local function increaseBoundaryRadius(amountVector) 
	local tmpItems = {} -- store tmp items
	for _, v in paris(boundaryFolder :GetChildren()) do
		if v.Name == 'boundPart' then table.insert(tmpItems, v)  end -- add only the found items to the list
	end
	
	-- now the nested loop, we 1st start with the generic loop then we loop through each item in the new list to change the size

	for j = 0, 5, 1 do
		wait(1) -- we could have the wait time as another param in the function so we can change it

		-- change all items in the new list
		for _, itm  in pairs(tmpItems) do
			itm.Size = itm.Size + amountVector
		end
	end
end

while wait(7) do -- this is "X" amount of seconds
    print("Starting")
    increaseBoundaryRadius(Vector3.new(0,0,5)) -- we pass the vector as its param
end

I hope this helps please comment if you do not understand how / why this code works.

Use spawns. They're like coroutines bit a little bit "easier". This allows multiple threads to be ran without yielding. Lua runs code in a chronological order, and loops often yield (unless you use coroutines/spawns), so in the end:

local boundaryFolder = game.Workspace.Bounds

function increaseBoundaryRadius()
    local amount = 5    

    for i, v in pairs(boundaryFolder:GetChildren()) do
        if v.Name == "boundPart" then       
	spawn(function()
            for j = 0, 5, 1 do
                wait(1)
             --this is where the parts should be increased every 1 second by adding 5 to their sizes

                v.Size = v.Size + Vector3.new(0, 0, amount)
            end
	end)
        end 

    end
end

while wait(7) do -- this is "X" amount of seconds
    print("Starting")
    increaseBoundaryRadius()
end

Read up if you want more info about it:

http://wiki.roblox.com/index.php?title=Global_namespace/Functions_specific_to_ROBLOX#Spawn