It is common to perform small-scale prototyping before large-scale application of three-dimensional (3D) printing technology and in this case size effects can have an important effect. There has been limited study to date on the size effects associated with 3D printing technology. Plaster and polylactic acid (PLA) were selected as typical inorganic and organic 3D printing materials. Binder jetting was used to print cube plaster specimens for compressive testing and fused deposition molding was used to print PLA beams for flexural testing. Five sizes were used for both plaster and PLA specimens. The experiments found that the compression strength of the plaster specimens decreased by 46% and the bending strength of the PLA specimens decreased by 15% and 39% relative to two printing schemes with the size increasing. Microstructural analysis using a digital image correlation technique were used to understand the specimen size effects observed. Existing size effect theories were used to model experimental results and recommendations are given regarding modeling efficiency. The paper provides experimental evidence on how the size of specimens produced by 3D printing technology influences properties and microstructure.