The effect of ruthenium particle size on Fischer-Tropsch synthesis (FTS) has been studied at 513 K, H₂/CO = 2 and 15 bar. Supported Ru catalysts with particle sizes ranging from 1.7 to 12 nm were prepared by using different Ru loadings and two different high surface area graphite (HSAG) supports to minimize the metal-support interaction. In addition, the effect of promotion with Cs is also evaluated. Microcalorimetric characterization during CO adsorption and XPS reveal a clear interaction between Ru and Cs. The FTS with Ru-based catalysts is, independent of the presence of promoter, highly structure-sensitive when the Ru particle size is under 7 nm. In this range the turnover frequency (TOF) for CO conversion increases with particle size, reaching a near constant value for Ru particles larger than 7 nm. Cs promoted catalysts display lower TOF values than the corresponding unpromoted samples. This somewhat reduced activity is attributed to the stronger CO adsorption on Cs promoted catalysts, as demonstrated by CO adsorption microcalorimetry. Product selectivity depends also on Ru particle size. Selectivity to C₅₊ hydrocarbons increases with increasing Ru particle size. For Cs-promoted catalysts, the olefin to paraffin ratio in the C₂-C₄ hydrocarbons range is independent of the Ru particle size, whereas it decreases for the unpromoted catalysts, showing the prevailing influence of the promoter.