Scale is one of the thematic threads that runs through nearly all of the sciences and is considered one of the major prevailing ideas of science. This study explored novice and experienced teachers' concepts of spatial scale with a focus on linear sizes from very small (nanoscale) to very large (cosmic scale). Novice teachers included undergraduates in science teacher education and students enrolled in a Masters of Arts in Science Teaching Program. Experienced teachers included students enrolled in a Master of Science Program. Participants' knowledge of conceptual categories of size, scale accuracy, and experiences learning scale were assessed. Results showed both experienced and novice teachers were most accurate in their knowledge of human scale (1 m or body length) and both groups were more accurate with large scale than small scale. Experienced teachers held more accurate concepts of small-scale measurements such as the nanometre than novice teachers. There was evidence that being able to directly experience objects and distances influenced concepts of size and scale. The role of in-school and out-of-school experiences in developing concepts of scale is discussed. (Contains 3 tables and 3 figures.)