Not exactly what you're after, but I've heard the guideline of 1/2 pound meat (main course) for dinners, then just proportion side courses appropriately. This might give you a rough starting point, which you could just proportion down depending on where you want this to fit between snack or meal.
As others have pointed out, time of day and other activities will affect how much people eat. Type of snacks can have quite a large impact as well; smaller, easier to handle items will go faster. You can even play with presentation to control how much people eat; placing small serving dishes out, and refilling them from a storage container will subconsciously cause people not to take as much, because there doesn't look to be as much.
Finally, your estimated numbers (50-75) is a HUGE gap. That's a 50% increase! With that large a gap, you're either going to have lots of leftovers, or no where near enough food. There's no way to plan for exact. So, plan things you'll like to eat for the next 3 days. ^_^
As for things to make...
Funeral/memorial snacks tend to be cultural. Are there any local foods or religious foods that are expected? Next, are there any favorite foods of the deceased? Of the family? Also watch for cultural taboos, and voluntary dietary restrictions: a meat-heavy spread in memorial of a vegetarian is probably inappropriate.
If you just want random snack items, most any appetizer would work. Look for things which can be prepared, stored, then served, as kitchen space gets to be a premium in the last hours before the event. Dips are nice, and stretch well. Wrapped items (like pigs in blankets, or boregs) are easier for guests to handle. You may even consider a roast of some sort and making mini sandwiches as a good-sized roast would stretch quite a way (hunt down slider buns, or buy/make small rolls). Picnic foods can be another great source of inspiration.