My family consist of two adults and two almost teenage kids. We currently have a 47ltr ARB fridge in the Patrol and a 65 ltr Isotherm fridge in the camper, which leaves us a bit spoilt for fridge space, therefore I can load all my drinks in the ARB fridge for the entire weekend away and keep the Isotherm fridge in the camper exclusively for food.
Before buying the camper we had a camp trailer which had a 60ltr Trailblazer fridge in it, which we used to use just for food and then also took an EvaKool esky (about 49 ltr) for the drinks.
I have also owned an Autofridge (39 ltr ?) which I found soon filled up and was only just big enough for our needs, hense the reason I started looking at the slightly bigger ARB one. I also considered the Waeco fridge of a similar size.
We had a great run out of our Trailblazer fridge, however found it a little rugged in its construction for keeping in the rear of the Patrol (bit sharp around the edges as well as a bit bulky).
60 ltrs of fridge space will be more than enough space for your family, especially with the ice box, and perhaps a smaller fridge (39 - 40 ltr) would also serve you well. I think too much space is better than not enough, and once you have made the expensive outlay for the fridge it is too late to say it's not big enough, but better to say it's a bit too big and can throw in a few more drinks.