Please Help newbie (Ground opening size for Bathtub Drain)

Hi all,
I had the bathtub framing done, Drain for tub is done,water pipe Hot/Cold done. pressure test for 2 days no Leak done. Now the time to framing the opening for the drain on the floor for the tub trap valve for service and clearance so I can cement it. Can any one tell what is the best size? I Measure 16"x16" is the best size for it. Please anyone Help. Thank you in advance....cheer

I think, I just found the answer. Its usually a 12x12 box cut out in the floor for service the trap valve. In case any one need to know...cheer

opening

IT only has to be large enough so the tub drain will fit into it. It has NOTHING to do with servicing the drain or anything else. Once the tub and drain are installed the opening is cemented closed.