Hi,
Gui slider actions have a left and right value, so you simply set the left value to the smaller scale, and the right value to the bigger scale, and you preset the slider knob value to 1 when you begin, that's about it. then you use the "set scale" action and inject in x, y z the slider value.
I have attached a working scene. Tell me if that's what you aim for.
Bye,
Jean