As a general rule of thumb, if you want one rug to cover the floor, choose one two feet shorter than the smallest wall in the room and try to leave an equal amount of flooring on all sides. But there are lots of options for each room. Using painters' tape to outline the space you want covered is an easy way to visualize the look and get measurements.
Measure your bed and add 24" to each side. Or put a runner on each side of the bed and one at the foot.
King size bed depicted
9' x 12'
8' x 10'
6' x 9'
3' x 5'
Runners on the sides should reach from the nightstand to the foot of the bed.
If using one rug, leave 18"-24" of floor on each side. Otherwise, furniture legs on or off the rug is your preference, and several smaller rugs to define spaces is also an option.
10' x 14'
9' x 12'
8' x 10'
The rug should extend at least 18" beyond the edge of the table so the chairs will still fit even when guests push back from the table (measure your table with all leaves in place). At least 18" of floor should frame the rug on all four sides. If possible, keep the rug small enough that buffets and china cabinets are not on the rug.
9' x 12'
8' x 10'
Leave at least 6" of floor showing all sides. A quick way to figure that: measure your hall, then subtract 12" from both length and width.
Be sure your runner extends the entire length of the hall, and you step on it, not bare floor, when exiting a room.