The microcontroller has 256 KB onboard flash for storage.
It is possible to attach a SD card to the SARA SFF.
Depending on how much storage you need you can go for the onboard flash or the sd card option.
Your question about battery size.
It all depends on how much your solution uses in sleep and when it's active.
* How long you will try to send data.
* How long the average GPS time takes.
* What your device consumes in sleep.
* Using an SD card also adds current consumption.
I would recommend the biggest you can fit in your box.
We sell up to 6000 mAh in our shop.
Also add a solar panel of minimum 1 WATT to keep your battery charged.