TweenSize just changes size with no effect?

The following code doesn't work, it just instantly appears.

wait(1)
script.Parent.RightBar.Size = UDim2.new(0.01,0,0,0.1) 
wait(0.1)
script.Parent.RightBar:TweenSize(UDim2.new(0.5,0,0,1),"In","Quad",3)

But this code does and actually tweens.

wait(1)
script.Parent.RightBar.Size = UDim2.new(0.5,0,0,1) 
wait(0.1)
script.Parent.RightBar:TweenSize(UDim2.new(0.3,0,0,1),"In","Quad",3)

Your issue is probably found on this line:

script.Parent.RightBar.Size = UDim2.new(0.01,0,0,0.1)

You are attempting to set an Offset value to a decimal. An Offset value is equivalent to a pixel on your screen and you cannot render decimal pixels. Setting this value to 1 will most likely fix your issue.