If you are using a simple staircase such as one of the "straight" options there is a workaround. Simply use the Sketchup "Scale" tool to horizontally scale the staircase giving you a smaller run.
For example if you require a run of 180mm, place the staircase with the 240mm run, switch to the scale tool and horizontally scale the staircase by 0.75.
You can apply this workaround to one of the "L-Shape" options as well... it is a little more complex in that you need to delve in to the component groups of the staircase and scale those individually. i.e. scale the first flight of steps then scale the second flight etc. You can do this in less than a minute or so.
Hope this is of use.