You should work full-size in Sketchup.
It doesn't "draw to scale".
Set you empty SKP model - Model Info > Units - to say 'meters'.
Import the site-plan image and place it down flat.
Use the Tapemeasure tool and pick two points on this image that you know to be a given distance a part - perhaps a road width or a building-frontage width - let's say it's 30m. The VCB will show a dimension [it won't be 30m unless you are the luckiest guy in the world! but that's not important], immediately type in the correct dimension as 30m and press <enter>, a dialog asks if you want to rescale the model, say 'yes'. Group it and lock the group so you don't accidentally change it. The image is now scaled to the right size, so drawing things over it at their true size will be in the correct relationships/proportions.
Now you can draw over it to add new roads and block-building forms, entering the sizes in meters as you want... Group or make components of these parts so they don't 'stick-together' when moving/rotating...
You can set some scene tabs of 3d perpsectives or 2d plan [no-perspective] etc with shadows on as desired.
Export these as images - you can scale a plan later - once your model is more complete...
