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Serialized messages in ROS are roughly the size that you'd expect them to be; that is, an int64 is 8 bytes, an float32 is 4 bytes, etc. Strings and variable-length arrays are serialized as a 4-byte length followed by the contents (no null terminator). Fixed-size arrays are serialized as a sequence of data, without the length. There is no padding.

I believe the rosserial protocol adds a bit of overhead because it transmits a topic ID along with each message; this should be well-documented in the rosserial protocol docs.