I have a 650 litre tank,
i only use half of that per day on some days,
on other i use more,
i simply went for the bigger option for when i got really busy and, or, when i have another guy working with me.
when i first went wfp, it was like this:
backpack, and constanly filling up with 25 litre containers.. (a pain)
then 250 litre baffled tank,
then 400
then 600
now 650.
i wish i had simply gone for what i have now, it would have saved me lots of hassle and money.
my advice would be go straight for the biggest you can carry,
think big cos your round never shrinks. (unless you purposely trim down)