The answer to that depends greatly on what you’re going to use the thermos bottle for. If you’re going to be using it for daily use, a smaller size may be more than enough. If you want to use it on longer outdoor excursions, a larger, more robust size may be in order.
If we took a technical stance to this question we would suggest you consider age and size, a larger person working outside all day will most certainly want a flask which can carry a good volume of liquid. A school student would take along a medium sized flask because he/she is only looking to drink at lunch hour or during a break from classes. There are mega flasks which are very good for group picnics and these flasks will carry a large volume of liquid which is generally enough for an averagely sized family.