To make sure that the locomotive and tender won't look too big or too small alongside the existing OO scale trains that my son has on his railway a card mockup of the Sketchup model has been made. Using the Flattery plugin it was pretty easy to unfold the model faces to create a flat printable mesh that could then be folded and glued.
This is scaled at 1:87 (which is HO scale rather than OO, but means that the train will be in proportion to the track width), and next to a small 0-4-0 steam engine looks about the right size. This card mockup will also be used to find a chassis to fit the locomotive, and I've already got donor parts for the tender chassis from an old flat bed carriage which has 6 wheeled bogies at each end (the tender is sitting on one of these bogies in the photo below).