The smallest one I managed to find was the Star Trek, Klingon Empire, B\'rel Class, 109 metres. I think valid ships need to have the text next to them. Some of them have poor image editing/cropping so there appear to be specks around some ships, such as above the text of the Praetor-II Class Battlecruiser, 4800 metres (also, the other very faint pixels around the same ship). See Red Dwarf for cropping.