How do i make a part longer on 1 direction?

I'm trying to make part longer but when i make the Z on size to 2 it goes 0.5 longer on both direction

You have two options:

1. Use :Resize()

There is a method for BaseParts called Resize that resizes the part on a certain face. However, if the part comes into a collision, it will move upwards & mess up the position. Something like this:

script.Parent:Resize(Enum.NormalId.Front,0.5)

will make the part 0.5 studs bigger on the front face.

2. Move the part after changing the size

However, I recommend just moving the position after you change the size:

local function Resize(Amount)
	local CF = script.Parent.CFrame
	script.Parent.Size = script.Parent.Size + Vector3.new(0,0,Amount)
	script.Parent.CFrame = CF + Vector3.new(0,0,-Amount/2)
end
Resize(script.Parent,0.5)

That just moves the part half the distance it's resized, keeping it in the right position.