I chose 500ml for a couple reasons:
In the USA, the regulatory bodies (TTB/FDA/LCB) dictate bottle sizes that can be used for the bottling of spirits. Although we typically use ounces in the US, liquor bottles are set in ML (1.75L "handles", 750ml "fifths"). I wanted the continuity with this system/standard.
As far as why I chose 500ml, I was frustrated with the small size of traditional hip flasks. Going through the trouble of pouring 4 shots into a tiny hole that was hardly shareable? No thanks... I wanted a better balance of volume and function without carrying around a water bottle or liquor bottle.