i use a medium tub, just about carry it full, holds about a bag and a half..
i tend to mix it so that if you get the whisk and draw a line in the mix and it stays there thats it...just so it dont fall off the toe when im startin the edges, any thicker and like the lads say it dont go as far, too thin and youll struggle to flatten it (and it'll be all over the floor!), too thick and youll have trowel arm just layin the ceiling on!
like peg says if you got a real bad wall youll throw nearly a full mix on just to try and level it...always take more than you think youll need, some surfaces that look reasonable for overskim will be miles out when you actually get on em...
windows and doors should be ignored when workin out materials/metreage etc...specially windows, you end up layin it on thicker round the beads so youll use more and it takes longer to skim round a window than it does to skim a flat area the same size as the window...more edges to clean, reveals to get straight etc...