This is one of those it depends on who you ask questions.
If you ask your condo association or a hotel most of the time you will hear “any dog weighing 25lbs or less”.
If you are talking to your local Kennel Club, they will probably categorize small dogs as either “Toy” or “Companion”. Older traditional thinkers stick to the “Toy” label while some of the more politically correct thinkers refer to the same dogs as “Companion”. The thinking is the term “Toy” gives the impression the dog is simply a plaything. At any rate, within this category there are many breeds. Among the largest is the Pug which on average weights in around 18lbs.
A sub group of the “Toy” category is the Teacup. This is most any dog under 4lbs. The Teacup is not recognized as an official breed. It is actually a runt that is born prematurely and often prone to health and behavioral problems.