For family size of 1-2 people 6-10 liter water geyser is enough if you take bath via bucket.
If you guys take hot water bath via shower then 15L is enough.
A lot also depends on how long is underground pipe from geyser to tap? How harsh winter is in area you live? Buy one bigger size of water heater if you live in hilly areas of India.
If your bathroom is on top floor of the house or an open bathroom? Here I recommend gas geyser as these need to be installed in open only. However buy only from renowned brand which can provide you service every year. Gas geyser need special care and maintenance. A yearly service from authorized service guy once a year at start of winter is enough for gas geysers.