I wanted to limit the range of possible values for sysvars (to hopefully make it easier for evolution), and I wanted it to make sense to humans. Which is where the -9999 to 9999 comes from. The stack can handle very large values. Either 64 bit or 128 bit, (I don't remember which). The numbers just get truncated when stored to memory. It isn't a memory issue, we can have bot memory be as big as we think we want it.
I've thought about fixed point numbers, never came to a decision one way or another.
For locations, I'm planning on not giving bots any absolute information. As far as the bot is concerned it's the center of the universe. If a bot wants to navigate around and then come back to its starting position it'll have to do something clever, like maybe leave a trail of chemicals behind to follow (haven't worked through exactly how this would work) or remember landmarks.