It all depends what you want (this assumes you have any choice in the matter!) but I would recommand a base with circa 6/7 a/c, i.e. not too small and not too large. Consider travelling arrangements as you will be going to and from the airport for 5 days at a time - how long will it take to get from your door to the crewroom and whether there is a dedicated staff channel (security) or if you will have to walk miles through a terminal building with masses of passengers! Also get some feedback about the Base Captain from those who are based there.
Hope this helps.