Despite widespread use of silk, it remains a significant challenge to fabricate fibers with properties similar to native silk. It has recently been recognized that the key to tuning silk fiber properties lies in controlling internal structure of assembled beta-sheets. We report an advance in the precise control of silk fiber formation with control of properties via microfluidic solution spinning. We use an experimental approach combined with modeling to accurately predict and independently tune fiber properties including Young's modulus and diameter to customize fibers. This is the first reported microfluidic approach capable of fabricating functional fibers with predictable properties and provides new insight into the structural transformations responsible for the unique properties of silk. Unlike bulk processes, our method facilitates the rapid and inexpensive fabrication of fibers from small volumes (50 mu L) that can be characterized to investigate sequence structure property relationships to optimize recombinant silk technology to match and exceed natural silk properties.