In this paper we outline a transistor size optimization technique for logic circuits that takes into account BTI (Bias Temperature Instability) and process variations. We demonstrate the accuracy of our results with statistical analysis. Since variations have a large impact on the scaling process, dependable circuit designs should include a quantitative analysis if they are to become more reliable in the future. In this study we used an algorithm to prove that with our technique we efficiently lowered the timing margin of the logic path by 4.4% below the margin achieved by conventional techniques. We also observed that the lifetime of the optimized circuits extended without any area overhead.