Grow up bins are typically very big, since you can put a bunch of weaners in together from multiple females. If you are feeding from hardware cloth on the top, and the tub is more than maybe 6 inches high, you can just throw a block of wood or an upside down food dish in the tub for them to use as a platform. Weaned rats are not exactly small, if you don't have experience breeding yet, you will see what I mean.