Actually they can attain the max size, but growth rates reduce considerably over time so it'll just take a few years or more if cared for properly. Taeniura lymma is rather difficult to care for, mainly due to new arrivals needing to be force-fed to prevent starvation and deworming. IME, 450g isn't large enough unless it is a custom tank and even then, prob. limiting yourself to a single ray with limited rockwork in the grand scheme of things.