Rare-cell isolation is an effective method for early detection and monitoring of cancer. To date, inertial microfluidic devices for rare-cell isolation lack tunability and high resolution due to their fixed geometries. Conventional inertial microfluidic devices are only able to separate rare cells with specific size ranges. However, cell sizes vary from sample to sample. We report the fabrication and evaluation of a stretchable microfluidic device for isolation of cancer cells of varied size distributions with high separation thresholds. The performance of the device was validated by separation of cancer cells spiked into white blood cells (WBCs) with a high recovery rate and purity.