This:
https://www.sparkfun.com/products/10138
Is your cell phone transmitter, can send the data via SMS.
This:
https://www.sparkfun.com/products/10922
Is your GPS reciever.
Put the two on the same board and give it power from a small lithium cell and you're good to go.
If you designed the boards from scratch with just the bare essentials from those boards above needed for your specific application, even smaller.
And if you ditch transmission by cell phone signal and go with something local-based, even smaller still.