How to Tween a Part's Size in a Single Direction?

You can tween both the size and position to give the illusion that it's tweening to one size instead of both.

The following script will resize a part to 20 studs long left/right depending if the part size is bigger or smaller than 20 without resizing the other size.

TweenService = game:GetService("TweenService")

part = script.Parent
newSize = 20

endSize = Vector3.new(part.Size.X, part.Size.Y, newSize)
endPosition = part.Position + Vector3.new(0,0, (part.Size.Z/2) - (newSize/2))

goal = {
    Size = endSize,
    Position = endPosition
}

timeInSeconds = 3

tweenInfo = TweenInfo.new(timeInSeconds)
tween = TweenService:Create(part, tweenInfo, goal)
tween:Play()