Tevo Tarantula I3 firmware
Answer for "2)":

The default firmware seems to be Repetier. It also includes Bed leveling (see documentation):
https://www.repetier.com/documentation/repetier-firmware/z-probing/

Marlin includes the Auto Bed Leveling feature too. You need to enable it by editing the Configuration.h file.

Bed size settings are there aswell.

"stylesuxx" made a video on how to get Marlin onto the Tevo:

                
            
or you can take a look at their website:
http://marlinfw.org/docs/configuration/configuration.html#configuration.h

Autoleveling can be enabeled as well in this file. There is a good video from Thomas Sanladerer on Mesh Bed Leveling:

                
            

If you want some help with auto bed leveling it would be nice if you can include what kind of sensor you want to use (inductive, capacitive, microswitch...) and what your prefered type of bed leveling is (mesh 3x3, just z-leveling)...