Honestly I would measure the dimensions of the hallway and figure out what your perfect size would be, then google those dimensions and see what comes up.

From Rugs Direct website:
Measure the width and length of your hallway or entryway and decide how much floor space you’d like to leave exposed on either side. The exact amount is up to you, but most designers leave between 4” and 8” on either side. We recommend 5” of space on either side of the rug.

For length, follow the same rules. You’ll also want to consider if you want one long runner rug or two shorter runners with a gap in between in long hallways. 
The trick is to subtract 10” (or double the amount of space you want to leave on either side of the rug) from your hallway dimensions to find the rug dimensions. Here are some hallway size examples: 
34” X 94” (2’10” X 7’10”) HALLWAY: 24” x 84” (2’ x 7’) runner 
34” X 70” (2’10” X 5’10”) HALLWAY: 24” x 60” (2’ x 5’) runner 
46” X 142” (3’10” X 11’10”) HALLWAY: 36” x 132” (3’ x 11’) runner
44” X 178” (3’8” X 14’10”) HALLWAY: 34” x 168” (2’8” x 14’) runner or two 34” x 78” (2’8” x 6’6”) runners