I think 8 x 10 rug is a good option to the size of the room 9 x 12 leaves no exposed hardwood on the sides which is not a good look imo.
9x12 rug under king size bed.
The rug will extend about a foot on either side of your king size mattress and out from the footboard about the same distance.
Depending upon the size of the room there can be more rug extension around the bed as seen in this room by lauren nelson design but not less.
The general rule for placement of rugs under beds is that a rug should extend 12 to 18 inches from the sides and foot of the bed 12 inches for a twin or full bed and 18 inches for a queen or king bed.
Just place half the rug under the bed leaving 4 feet at the front of bed for your bench.
For an area rug to look balanced under a bed it should be large enough to extend beyond the sides of the bed at least 18 inches for a king or queen bed and at least 12 inches for a full or twin bed.
Use an 8 x 10 rug horizontally under your bed.
Below we ll discuss several different ways to place a rug under a king bed.
If you find a pair of rugs you like in this size you can place two 4x6 rugs under a queen bed.
Runners on each side.
Place a rug horizontally.
This configuration would take up more space than a single 9x12 rug which could be useful in some circumstances see below.
For a symmetrical finish an 8x10 rug is best for queen beds and a 9x12 for king.
Center a 9x12 area rug under a queen or king size bed to make the room feel larger and to add softness underfoot.
9 x 12 twin beds in a bedroom with a one twin bed select an area rug that is large enough.
A rug that is too small will detract from grand look and dimensions of the entire room.
Make sure that the rug itself doesn t slide.
A general rule of thumb suggests that an area rug should always extend at least 18 inches to 24 inches from the sides and from the foot of a queen size and a king size bed.
As far as the under bed rug i like pelts either natural or faux instead of a rug that goes all the way under the bed.
A 9x12 rug is a great option as it works great with a variety of bed and room sizes.
Ways to place a rug under a king bed.
This way it is easier to clean under bed especially for a teen boy.
8 x 10 or 9 x 12 king size bed remember the bigger is better rule when it comes to arranging an area rug under a king size bed.
For this you can use a padding for rugs and even special grips that they sell for the padding.
A 12 foot by 15 foot rug will look balanced under the longer california king size bed and this size area rug will even fit under nightstands.