Hi Árni,
Which type of light are you using (e.g. point light, area light)?
Point lights’ sizes are determined by their ‘diameter’ setting (in Shaderlight 2017 – in earlier versions of Shaderlight their diameter was always zero) and its size in the SketchUp viewport has no bearing on its size in the render.
Area lights can’t really be scaled after creation – you should try to create them at the right size to begin with. It is possible to scale them afterwards by putting them in a group or component and then scaling that group or component, but we don’t recommend that because it can cause the light given off to have an incorrect brightness and orientation.
I hope that helps,
Shaderlight support