You would need to be careful - in many states it is against the law to record other people without notifying them that you are recording.

What would be some ways of going about building an extremely small, inconspicuous, and reliable personal recording system?

I would think you could almost use a phone with some extra hardware if you could find some ways of offloading or reducing battery consumption. Then it's just a matter of streaming it to the cloud.

i think google voice does something like this(only for phone calls).

If google can take this and make it usefull and profitable , the distance to recording everything you say is not long.