I wonder how your lecturer would mark this if you submitted your assignment with the reference (Harvard reference style guide etc) of "Pprune" for this information!
A quick Google of "B777 interior volume" brings up quite a number of useful websites but in any case I don't think any airline uses interior volume as a metric in determining seat numbers or unit costs. Interior floor space for certain, maximum seat pitch determined by class of service; space allocated to toilets, galleys, emergency exits, width of available cabin- all these things yes but total cabin volume? Unless of course you are Ryanair and plan on putting in a "sardine" class.