This paper discusses the definition of simple dimensioning rules for high speed IP access networks carrying data traffic. We notably provide formulas relating capacity, demand and performance allowing dimensioning for a target quality of service expressed in terms of useful per-flow throughput. These formulas derive from a data traffic model equivalent of the Engset model for telephone access networks. Performance is shown to be largely independent of precise traffic characteristics. The key dimensioning parameter is offered traffic defined as the average data rate a user would generate in the absence of congestion.