I fly a 1969 model C172K. I'm sure different models have different dimensions. I measured my back seat area and cargo area earlier this week. Cargo area was 19"L X 29"W X 30.5"H. Length is measured from the back of the backseat to the aft bulkhead. Width is the width of the cargo area from side to side. Height is measured from the floor of the cargo area to the back window just above the rear bulkhead.
Backseat measured. 27"L X 35.5"W X 32"H. Length was measure from the backseat to the back of the forward seats when they're moved forward and engaged in the first seat stop. I could have moved it some more forward but I'm a fairly tall guy. Width is cabin width. Would have been about 3" wider except for the armrest on each side in the backseat area. Height is from the front of the backseat to the headliner.
Cessna 172/Huntsville, AL - Moontown Airport (3M5)