As a professional it is very difficult to explain on how to size drains properly as when we go to trade school, we learn about the codes and practices in the codebook and on the job we learn sizing on a day to day basis being on the job under the direct supervision of Journeypersons.
For the homeowner, a basic knowledge of the minimums that we find in the trade is always a good idea however. When faced with issues a homeowner can be of great information to the trades over the phone before we even get to the site. Also, we as professionals already have a baseline as to what things should and can be, but even we can be taken by surprise once and awhile.
For your standard basin or lavatory sink, the standard sizing is 1 1/2". This is for a single sink. Sometimes the piping can join below the cabinetry and therefore the pipe sizing will increase. A secondary thought is when the basin is used as a wet vent, the sizing can be 2".
With Bidets, again the normal size is 1 1/2" , and normally does not change. Bidets are a wonderful fixture, although some plumbers will never encounter them. A smaller part of that same pool of tradesman will never see them at all and can be a very strange thing to work on.
Toilets are 3" standard. In certain cases the drains can be 4" although this occurs in only a few cases. What our experts normally find is in this case, it is in an older home and installed back when it was the plumbing code.
Kitchen sinks, both single and double, are sized at 1 1/2". When there are specialized fixtures such as a garburator and such appliances, a drain as large as 2" can be installed. The most common problem with a kitchen sink is a small leak or a blocked drain.
Bathtubs are sized at 1 1/2" standard. This includes the part called a waste and overflow and the trap. With certain bathtubs , jets, pumps, motors and the like, the drain could be as large as 2" and even beyond. The largest problem with bathtubs is normally a blockage. This is caused from a buildup of soaps and hair clogging even the best-maintained drains.
When we as professionals are faced with specialty fixtures and out of country appurtances, we can never really know what we are dealing with. Sometimes with this high-end stuff, the plumbing can get very difficult to adapt to the Canadian standards of connection. Rarely does it become necessary for us to replace a fixture but more often than not, to order parts and materials that just are not available here. With this in mind, sometimes repair parts can be days or weeks away and for purposes of avoiding the problem in the future, we normally get more than one replacement part at a time to help keep the down time to a minimum.