It's going to change slightly based on the build, but not by much.
Engine no exhaust or heater boxes:
Approx 32" wide, 25" tall, 17-18" deep. This is with flywheel/clutch/pressure plate installed, it's about 17" to the edge of the front crank, 18" to the nose of the alternator nut I'm using. Exhaust will increase the depth measurement, and will also increase the height measurement.
Transaxle IRS, from a '72:
13" high, 15" wide, 27" long.
All measurements above eyeballed with a tape measure on parts that are not installed in a car.
-Dave