For some thats the life size tank, but I would say a 240 absolute minimum for life. But you could keep it in that for a long time, til its a little more than the width then you would know its time to move it.
Of course then you have the problem of finding a safe way to move it once its so large.
Not always. My last silver grew from 6" or 7" to a tad over 24" in a little less than a year! It grew out to that size in a 240 gal. The fish was then moved to a 600 gal. tank. It can be amazing how fast they grow with good care.