The 240: 4.00" bore x 3.18" stroke. 240 cubic inch displacement. Large six family (used in fullsize Fords and Pickups) along with the 300 (4.00" bore x 4.00" stroke).
The 250: 3.68" bore x 3.91" stroke. 250 cubic inch displacement. Small six family (used in compact and midsize Fords, like the Falcon and Mustang) along with the 144, 170 and 200.
The 250 is smaller EXTERNALLY, but has MORE displacement.
The 240 is larger EXTERNALLY, but has LESS displacement.