A sensible heater size for your tank, from its volume and how much warmer than the room you need to keep it.
A heater has to replace the heat a tank loses to the room, so what matters is not the tank size alone but how far above the room you hold it and how cold the room gets. Around 1 watt per litre is a fair starting point for a few degrees of lift in a normal room; a cold room, an open top or a conservatory need more. If you are unsure of your true volume, work it out first with the volume calculator, and see the electricity side in the running cost calculator.
It depends on the volume and how far above room temperature you keep it. This calculator sizes it from your volume and the gap between your target and the coldest the room gets, then rounds up to a standard heater.
About 1 watt per litre for a few degrees of lift in a normal room, more in a cold one. The calculator works it out from your actual temperatures rather than a flat rule.
On tanks over about 200 litres, yes. Two smaller heaters give redundancy if one fails or sticks on, and more even heat.
A guide, not a precise figure. It scales with volume and with the gap between your target and the coldest the room gets, then rounds up to a standard heater size. A well insulated room, a lid and a covered tank all reduce what you need; a cold room, an open top or a conservatory increase it. On tanks over about 200 litres, two smaller heaters are safer than one big one: you get redundancy if one fails or sticks on, and more even heat.