Size wise, they are very close. The P64 is thinner, but not by a large margin. I measured using a plastic digital Harbor Freight caliper so I won't pretend it's absolutely exact. The following is my opinion and you know what they say about those.
Slide Width
P64 = .82 of an inch (not including safety)
P64 measured at safety = .93 of an inch
P11 Slide width = .99 of an inch
Grip Width
P64 with Marschall grips = 1.12 inches
P11 grip = 1.03 inches
Length
P11 approx 6"
P64 approx 6.25 inches
Height with magazine
p11 = 4.25 inches
P64 = 4.30 inches (Marschall grips)
Shooting Experience
P64
The P64 has unpleasant recoil. I fired 50 shots, and that took some effort. I was cut by each side of the side. I had tried to maintain a very firm grip, but after 2-3 shots my hand rode up enough to get bit. It wasn't a bad bite and it took a few magazines after I had been bit to realize it. My accuracy was only ok at 21 feet. I'm used to shooting out the center of the target and leaving a quarter to 50 cent piece sized hole. That didn't happen. This is in spite of installing a 20lb recoil spring and 19lb hammer spring. The tiny sights did not help either. A point in the P64's favor over the P11 is the Marschall grips. It allowed me to have all my fingers on the grip which helped with control.
P11
The P11 has a short grip thanks primarily to the location of the trigger guard, and even my small hands had the pinky hanging off. The DAO trigger pull is long and heavy. Recoil is significant, but managable. The sights are better than those of the P64, but they are plastic. Part way thru shooting one of the white dots on the rear sight fell off making shooting a bit more interesting. My accuracy with the P11 was worse than that of the P64. However it is a 9mm parabellum caliber and it holds 10-11 cartridges to the P64's 6-7 of 9x18. The pinky extension for the magazine may be a good investment.
Pros
P64
More accurate
Better grip with the right grips
Plenty of room for your finger in the trigger guard
Once modified, the single action is good
Cost (cheaper than the P11 )
Cons
9mm parabellum recoil from a 9x18 cartridge
Heel style magazine release
Slide mounted safety
Cost/availabilty of spare parts
Magazine capacity
Pros
P11
Better sights/picture
Higher magazine capacity
Traditional American magazine release
Cons
Trigger has long, heavy pull
My hands are a little larger than that of a small woman's and there isn't much room for my finger in the trigger guard
The skinny trigger irritates the trigger finger upon firing
I shot worse with it than the P64
Plastic sights that aren't durable enough
Plastic guide rod
I had to buy the metal guide rod and the trigger shoe. The metal guide rod replaced a chewed up plastic one, and the shoe made the trigger marginally better. I still had to take a dremel to the shoe so it wouldn't irritate my trigger finger.
Replacing the rear sight on the P11 is no fun either. I would consider the P11 a close range, carry a lot, shoot a little gun. The same with the P64.
