A method for in situ testing of miniaturized tension specimen was developed. The size effects of the plastic deformation behavior of copper single crystals loaded along the <-2 3 4> direction were investigated. The diameter was varied between 0.5 [mu]m and 8 [mu]m, and the aspect ratio, gauge length to side length, between 1:1 and 13.5:1. At high aspect ratios hardening was negligible. However, an increase of the flow stress with decreasing diameter was observed. This increase was small for diameters above 2 [mu]m, and somewhat larger below 2 [mu]m. These findings are explained by individual dislocation sources which govern the plastic deformation. For low aspect ratios the behavior is significantly different. A pronounced hardening and a very strong size effect was observed. Both are a result of dislocation pile-ups due to the constrained glide of the dislocations caused by the sample geometry.