For tank size you have to change "Tank Size"
Config menu->Adjust->Tank size
Where did you find "TANK FUEL"?
And you should change something if something is not working.
Do not change if everything is ok.
And yes - reset tank after you fill it (full).
There is no manual - it is DIY project and it is assumed that
obduino32K.txt (in google trunk near obduino32K.pde) is enougth. And comments in obduino32K.pde are self explaining.
Plus here we have 75 forum pages with most issues discussed. For all new with OBDuino it is a must to read it all.
It would be great if somebody could write documentation and maintain it, or fill
opengauge - Open Source Fuel Efficiency Instrumentation - Google Project Hosting wiki page.