A pi zero running wifi should use under 2W. Powering an RTLSDR dongle takes about 300mA or another 1.5W or so. If you assume an power budget of 5W, that should be plenty for continuous operation.
That gives a total of around 120Wh that you need to supply each day. If you are using a 12V battery, that means a continuous draw of just about 410mA ignoring any voltage converter losses. That’s approximately 5 Ah of battery capacity. That’s quite a conservative estimate, and in reality you probably won’t need quite that much.
You want sufficient solar capacity to maintain a battery at a healthy voltage, such that if you get a day or two of poor weather and low output it has enough reserve to keep operating. I’d estimate that you probably want something like a 20Ah or so battery. Solar panels do not put out anything like their rated maximum output when averaged over a day, but this varies hugely depending on where you are. Somewhere in higher latitudes will require a larger panel than somewhere where it’s consistently sunny, especially accounting for seasonal variation and short winter days.
It would probably be a good idea to use something like a Sleepy Pi 2, which can take a 5.5-30V input to power the pi and gives options for power monitoring and a RTC.
This article has some thoughts on powering a pi with a solar system.
Edit: Since this post, I have bought and installed a Dockerpi powerboard. This takes a ~12v DC input and feeds 5v to the pi via the pin header. It can supply up to 5A at 5V, so is plenty to power pi, dongles and an LNA.