In order to answer this question which many have asked before, keep the following questions in mind.
What will you keep in the flask? If you are carrying water or sports liquid, you may want a larger flask than if you are carrying alcohol. If the primary purpose is replenishment of fluid lost, you are probably going to want a larger flask. If the primary purpose is carry alcohol, you may not need something quite as large.
How many will drink from the flask? If the flask is for you alone, you may need a smaller flask than if you intend to share. How much will you drink?
Where will you keep it? Does the flask need to fit in a pocket, purse or other container that may dictate the size of the flask?
Is the flask a gift? If so, what would the recipient’s requirements be (see questions above)?
Do you want the flask engraved? If so, some flasks have one vs two or three lines of engraving. Typically, the larger the flask the more you can have engraved on it. Some of our flasks have the ability to engrave three lines with twenty characters each, others allow only three characters.