Not forcing you to carry around a size is a feature, not a bug - if you don't need the size, it's just a waste of space.

In C, the size of the array is there even if you can't get at it. On the stack, the compiler needs to know how big an array is to know where to put the next variable; on the heap, the runtime needs to know how big the buffer was returned from malloc so it knows how to delete it. You just don't have access to that number.

I had heard that the museum was "small but pretty interesting". That ended up definitely being an under-sell.

The Computer History Museum in Mountain View is cool and all, but the Cryptologic Museum struck me on an entirely different level. Instead of the "Here is how computing evolved" theme of the Mountain View museum, I really felt like this was the "Here is why computation is relevant to communications (and warfare)" counterpart. They display voice and data encryption tools of the last five decades, from STE's and STU-III's back to (as other posters mentioned) the mechanically-synchronized SIGSALY machine that used giant turning vinyl records to encrypt the traffic. There is a handset you can pick up to hear pre-recorded messages representing the voice quality of each system. The oldest were barely intelligible, the newest are (obviously) crystal clear.

The Cray XMP and YMP are impressive, and are in almost flawless condition! Rather than the exhibit at Mountain View, it felt like these machines were just recently taken out of service, and could easily be made operational again. They didn't seem like they'd been cobbled back together or had sat in closets neglected and falling apart for years. The density of some of the components on the Thinking Machines CM-5 memory and processor slices is impressive, and the descriptions of the power and cooling apparatus required (think many kilowatts and lots of Fluorinert) were equally amazing -- truly a testament to what can be done when money isn't much of an object, and a machine's value is measured solely in MIPS or MFLOPS.

There is a three-foot-tall full-relief wooden replica of the Great Seal of the U.S. on the wall, which apparently was a gift from Russian schoolchildren to the U.S. embassador in Moscow. After hanging prominently on the wall for years in the embassador's office in Moscow, in 1952 it was discovered that it contained a resonant cavity eavesdropping bug on the inside that was very difficult to detect with sensing equipment of the time, unless it was activated by radio signal (presumably by Soviet spies) from the outside. I met there three (very proud) tourists of Russian descent who chuckled heartily at that one (and who tried to teach me how to say "Medvedev" properly, thanks!)

As everyone else mentioned, the working Enigma machine was fun to encipher a message to a friend with (they have a pad and pencil for you to use), and the displays on the history of the agency and of the Korean and Vietnam conflicts were well put together. The GRAB II and Poppy ELINT satellites were especially interesting to me, and reminded me of the kind of things a senior class at the USAF Academy might build for a project these days (relics of an era when launch considerations and electronics density actually drove simplicity into designs).

If you're an electronics/history/information assurance/security/aerospace/DC trivia fan, you'll almost certainly enjoy the trip, even if the facility is kind of small and out of the way. While you're in the area, go see the Udvar-Hazy center, too! And don't forget to tip your docents...