Size.....your on drugs man.....the cars ARE the same size!, the just look bigger in the 67/68 because they are more rounded.
Other Give aways....VERY had to imitate!.....
Wiper Vents. Has two sets left and right, with a ridge through the centre.
65/66 has 6 smaller sets in squares, with no centre ridge.
Check the link below, this car is a 65.
Mustang Owners Club Inc. Victoria
Also while on the Scuttle area, on the outers, 67/68 has a nice curved slope down to the guards, whereas the 65/66 has a sharp step. As such, a 67/68 bonnet will NOT fit a 65/66, and the reverse vents were not available on the 65/66
Then you get to the sides, looking how the line runs down the guard, along the door, almost vertical down the quarter, and then part way forwards again along the door. On the 65/66 this lower line runs all the way back to the front wheel. On the 65/66 this 'scallop' is not as deep as on the 67/68. Then you have the 67 which has false vents, which this car does not. After 68, the changes were not so subtle.
Not shown in the original posters post, is a pic of the Rear Quarters, a 68 also has reflectors just in front of the rear bar.
All that on top of the obvious differences in headlight buckets and grille openins.