Work out the minimum continuous and surge rating you need from a 12V or 24V inverter, based on what you'll actually run off it.
Sizes are rounded up to the nearest common inverter rating (300W, 600W, 1000W, 1500W, 2000W, 2500W, 3000W). Always check your specific appliance's rated wattage on its data plate, and size your battery cabling and fusing for the DC current shown below - an inverter drawing its rated watts from a 12V system pulls roughly 8x the amps a 24V system does for the same job. This is a planning estimate, not an electrical design - for anything permanent, have a 12V auto-electrician confirm cable and fuse sizing.