Well, since I am a little slow in getting to these posts, I will just have to say that I concur...
However, most home building supply warehouses (i.e. Lowes, Home Depot, etc.) carry in stock actually 3 different "standard" size tubs. That is:
30X54
30X60
32X60
Now that being said, the most common size I have seen in my 17 years ofo residential construction is the 30X60, although I have seen some builders that would not use anything except a 32X60.
There are also other size tubs available from nationally known manufacturers typically ranging in 6" increments in length from 54" to 84" and widths from 30" to 36" in 2" increments.
just my .02 worth