Welcome!
In terms of pinky-nail size

for 5 crabbies I would go with 6-7 pieces, to start off with. If they eat all of that, go ahead and offer more next time, if they dont you can go to 5-6 pieces. Some days they will be more hungry than others, so youll have to watch and see what they do.
You dont have to give them new food every day, mine dont eat every day. Ill offer food and then wait 2-3 sometimes 4 days depending on what is left and if I see them out foraging..then I will put in a new dish of food. Feel free to leave the food for 2-3 days, or until it molds--then you want to remove it and replace it with new.
Water you can change when ever you want, I have deeper dishes so I just refill mine unless there is a ton of dirt in them, thenI just scoop out the dirt and fill it with water. But as long as they have water and its not rotten/nasty smelling/looking (mine like to add all sorts of things like food to their water sometimes..which makes it a bit raunchy and then I will do a full wash the dish refill water change), its all good
