If I remember right, it's the measurements that he went over in Planning the Built-In in the Planning section.
It came from the measurements he took where the cabinet would go, combined with already known dimensions of the materials he'd be using and the allowances needed for various things.
Hi Eden,
As Grady said, I go over the dimensions for the cabinet in the previous lesson, where we plan the built-in. Materials like the 18mm plywood are standardized.
As for the decision on how big to make the individual cabinets, I think I went over this, but it's a mix of talking to the client and respect their wishes and finding a good design balance. For this project it felt like a good balance to have the wider cabinet in the middle and evenly sized cabinets surrounding it.
Then take into account that you need space to walls (wonky walls, opening range of doors etc.) and see what overall dimensions make sense and if you can get some round numbers out of it. Personally I prefer having cabinets at for example 700mm total width if possible, rather than 693mm or something crooked like that. It usually takes some calculating around until you find the right dimensions, but in the end it does make it easier for production. :)