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The size of an object is key length + data length + data structure + a stringified "flags" field + 2 bytes. Plus or minus two bytes since I'm recalling this from memory :P
Note when I say "data length", for you that would be the serialized (marshalled/json'ed/etc) value before rails puts it in memcached. If you know the key, you could fetch it once by hand to get the length back.
You can find the data structure size by running the "./sizes" command out of a memcached tarball after running "make".
The total bytes in the system is "curr_bytes" as resulted from the "stats" command, but that doesn't account for flushed or expired items still in the cache.