Check Object Info dialog
Use the Window > Object Info (Command-I) menu item to open the Object Info dialog. Notice the numbers in the Rotation Y and Scale Y edit fields. These are the numeric equivalents of the transformations (scale and rotation) you just manually applied to the cube. (Your numbers probably will not match those shown here.)

Manually set transformation
To reset the cube back to its original size and orientation, simply change the X, Y and Z values for the Rotation, Scale and Position to match the settings in the diagram below.
NOTE: To apply your new settings, enter the value and hit return or enter.