From experience: Buy a popular size oven if you will have a built-in. They are 60cm and 90cm, nominally a few mm smaller, but read on. There are a few 70, 75, and 76cm models on the market, but if you need to replace them you want to be able to buy one that fits, and some sizes seem to be getting retired from the market so choice can be limited or absent.
Our oven is a 75 that is actually 74.5cm. The cabinetry is made for 74.5. There are no 74.5cm models on the market now. Replacing it will be a 70 as best fit.. As I previously posted on another thread, there is no way to make my oven cavity larger. I suspect at one time 75cm meant the hole size, while now it means the oven size. If I had to do it again, and needing capacity and footprint on the shelves, I would do a dual oven 60cm.
If the fridge is being assigned a place with side and top cabinetry, fridge sizes also change over time. We did a market survey of every fridge on the market in 2011 and had the fridge space built to accommodate almost any of them. Guess what. Today's fridges (in our preferred size/feature range) are mostly taller and will not fit.
The message is to plan as if you will replace all your major appliances and make space available for contingencies. If a cavity is larger than necessary it can be framed in for aesthetics, but if it is too small and cannot be enlarged, too bad.
My partner designed our kitchen to perfection - until we met with the evolution of appliance sizes.