In a dining room, the rug should be large enough for all the chairs to be on the rug. In a living area, the rug should cover much of the area where furniture is located. Ideally, the front legs of the furniture would be on the rug. Some people like to put all the upholstered pieces entirely on the rug, but I think that front legs are enough and sometimes not even those.

In an entry, there are too many variables, but I will say that the rug should not look like a postage stamp and should not be pushed up to the front door. I prefer not to have the legs of an entry table on the rug, though others don't mind.

The rug in a bedroom should be far enough under the bed so that when getting out of bed, your feet would hit the rug and not a cold floor. It doesn't have to be under the night stands, but should continue beyond the foot of the bed for about 3' or more.
Carolyn Albert-Kincl, ASID

Great idea book for you:
https://www.houzz.com/magazine/11-area-rug-rules-and-how-to-break-them-stsetivw-vs~222609