A 6'x9' rug is a good choice for a smaller-sized room that cannot fit an 8'x10'. This size is less commonly used, and there is less of a selection in the rug world as far as style and design. Most 6'x9' machine made rugs are actually closer to 6.5'x9'5' therefore occasionally called 7'x9' rugs.
A 6'x9' rug comfortably accommodates a table with 4 chairs allowing enough room to keep the chairs on the rug when they are pulled back for sitting.
In a bedroom with a full sized bed a 6'x9' rug provides an 18" border on each side of the bed. This size is recommended for smaller rooms that cannot handle an 8'x10' rug.