I have an older sea to summit 4L bladder. Not often for every day but useful in those cases where I am going through arid areas and need extra water. Crossing Russia my riding partner had a 10l bladder shown in the image below. Towards the end of each day, we would find our way to a village or other habitation and ask where we could fill on water. Sometimes it was a well, sometimes a pump house, sometimes they went into their house and came back with it full. After that with water bladder strapped on back, we would cycle perhaps another 10km and then find a place besides the road to camp. We would use the water for cooking, washing up and then filling bottles for the next day.
When cycling around Australia I had some dry regions and my general guideline that worked for me was ~1 liter for each 20km and ~2 liters for an overnight. At maximum there was a gap of ~280km going to the Sandfire Roadhouse. This meant 16 liters split as follows: Camelbak (3l) + Camelbak 2nd bladder (3l) + bike bottles (2l) + sea to summit bladder (4l) + extra water bottles (2l) + spare bottles of water purchased (2l). As it turned out when I camped, there were people offering me water and also at least one that stopped along the road. So my 16l was more than enough but ok for just in case...