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I will present a new method to measure the color-dependent source size in lensed quasars using the microlensing time delays inferred from multi-band light curves. In practice, the relevant observable is the differential microlensing time delays between different bands. I will show from simulation using the facility as Vera C. Rubin Observatory that if this differential time delay between bands can be measured with a precision of 0.1 days in any given lensed image, the disk size can be recovered to within a factor of 2. If four lensed images are used, our method is able to achieve an unbiased source measurement within the error of the order of 20%, which is comparable with other techniques.