The generation of low-emittance electron beams has received significant interest in recent years. Driven by the requirements of X-ray-free electron lasers, the emittance of photocathode injectors has been reduced significantly, with corresponding increase in beam brightness. This has put increasingly stringent requirements on the instrumentation to measure the beam size. These requirements are even more stringent for novel accelerator developments, such as laser-driven accelerators based on dielectric structures or on a plasma. We present here the generation and measurement of a sub-micrometer electron beam, at a particle energy of 330 MeV, and a bunch charge below 1 pC. An electron-beam optics with a vertical beta-function of a few millimeters has been setup. The beam is characterized through a wire scanner that employs a 1 mu m wide metallic structure fabricated using the electron-beam lithography. The smallest (rms) transverse beam size presented here is <500 nm.