Discontinuous polystyrene thin films adsorbed on graphite surfaces from dilute solutions were imaged directly by scanning tunneling microscopy (STM), without the use of conductive overcoating. Dimensions of the adsorbed PS structures were evaluated by measurement of the STM images. At low surface coverage (<10%), the adsorbed polystyrene structures were found to be thin, flat "micro-islands", which are typically composed of 3-50 polystyrene molecules. Monodisperse polystyrene standards of molecular weights ranging from 7000 to 3 000 000 were used to investigate the effect of molecular weight on the micro-island dimensions. The smallest of the imaged structures were identified as single polymer molecules. These single-chain structures have elongated, asymmetric shapes similar to those predicted by statistical theories. The mean square radius of the single-chain micro-islands was found to vary with molecular weight raised to the exponent 1.24, while the apparent thickness was found to depend only weakly on molecular weight.