Serological assay for coronavirus 2019 (COVID‐19) patients including asymptomatic cases can inform on disease progression and prognosis. A detection method taking into account multiplex, high sensitivity, and a wider detection range will help to identify and treat COVID‐19. Here we integrated color‐size dual‐encoded beads and rolling circle amplification (RCA) into a bead‐based fluorescence immunoassay implemented in a size sorting chip to achieve high‐throughput and sensitive detection. We used the assay for quantifying COVID‐19 antibodies against spike S1, nucleocapsid, the receptor binding domain antigens. It also detected inflammatory biomarkers including interleukin‐6, interleukin‐1β, procalcitonin, C‐reactive protein whose concentrations range from pg mL−1 to μg mL−1. Use of different size beads integrating with RCA results in a tunable detection range. The assay can be readily modified to simultaneously measure more COVID‐19 serological molecules differing by orders of magnitude. [ABSTRACT FROM AUTHOR]