I've had a Santro GV from 2005-2010 too.
Not sure about the cylinders' size but it was the same factory fitted 3-cylinder setup. Average mileage within the city was 140 on normal days and 110 on days immediately after gas load shedding (blame residual air for that). If you're getting above 100 in the city, there isn't much need for immediate attention.
Filling capacity is a little over 7Kg so that's perfect too. No need to get the cylinders cleaned up, the residual oil barely takes up any volume and shouldn't be an issue unless it clogs up your cylinders' nozzles. It's best to get your car tuned up from an authorized dealer if you must.