The creation of an FPGA requires extensive transistor-level design. This is necessary for both the final design, and during architecture exploration, when many different logic and routing architectures are considered. For such explorations, it is not feasible to spend significant amounts of time on transistor-level design. This paper presents an automated transistor sizing tool for FPGA architecture exploration that uses a two-phased approach - a coarse rapid phase with simple modeling followed by refinement with much more accurate models. The output of the system is a design optimized towards a specific area-delay criterion. We compare the quality of our results to prior manual and partially automated approaches. Also, our tool has been used to produce hundreds of candidate architectures which we are releasing to support future high quality explorations. Categories and Subject Descriptors: B.7.2 [Integrated Circuits]: Design Aids General Terms: Algorithms, Design, Measurement