The present invention relates to a bioreactor assembly containing a reactor frame that permits linear upscaling under identical conditions of culture volumes from two liters to thousands of liters. The bioreactor contains at least one rocking platforms being capable of holding at least one disposable bag adapted for receiving a total volume of at least 2 liters. The at least one platforms have a tray-like shape, that induces the fluid in the bag to move and form a wave-like motion. The at least one platforms are attached to the reactor frame and are positioned on top of each other along a vertical axis. In particular, the weight distribution of the rocking platforms is approximately in balance during rocking. The present invention further relates to methods using the bioreactor assembly.