how do i prevent a bilboard gui from expanding too much?

script.Parent.AbsoluteSize = 0,200,0,50
script.Parent.Frame.AbsoluteSize = 0,200,0,50

I don't think you need a script to change a billboard gui's size if it's already in the workspace. If it is in the workspace, then the frame inside the billboard gui has to have a size of {1,0},{1,0}. The billboard gui has a size of its own and you have to use the numbers on the left {3,0},{3,0} to resize it. If there is numbers on the right {0,3},{0,3} then it will grow the more you get away from it.

If you were to use a script then use this one.

script.Parent.Frame.Size = UDim2.new( 1, 0, 1, 0)
script.Parent.Size = UDim2.new(yousize,0,yoursize,0)