Window sizes and interior doors

No, that's not how it is. Take a look at, for example, Bauhaus.Info.

No one buys standard size windows for a house at the hardware store, and there is no price advantage in doing so.

Exactly, when you have to save, it's still enough for cheap providers from Poland. Bauhaus windows will probably never see a single-family house, for good reason.

You smart aleck. Original conversation with our BU, when the house was planned but we still wanted to adjust the price: Let's take standard windows. I have a Polish supplier who always keeps them in stock, he also supplies hardware stores. That's good quality at a good price. Saves us another 2500 on the house. That was his suggestion. And the result is that we could even take the standard pleated blinds from the Danish bed store, they fit.

Reached the point to define windows, patio doors, and interior doors. Are there certain standard dimensions,

The standard dimensions are derived from the construction guideline dimensions. Facade windows are market-standard custom-made, as described, not least motivated by tolerances. For interior doors, this is accounted for in the construction, so standard storage sizes are common here: nominal construction guideline dimension 201 (modern 213.5) in height and correspondingly 88.5 cm in width for standard room doors (101 cm considering walkers), 76 cm for storage rooms and partly also for bathrooms, I would dimension front doors at 113 cm (storage size 110 x 210).

No one buys standard-size windows at the DIY store for a house, there is no price advantage in that either.

Windows at the DIY store are hardly bought only by dogs and horses or for garages and garden sheds. Of course, there is a price advantage to manufacturing standard sizes by the pallet. That way the DIY store still keeps a margin.

There is of course a DIN for Aufenthaltsräume. Or whatever it is called. It's about light. Just google it, you’ll find it quickly. However, I would always take twice as much space as necessary. And then it depends on the room layout and direction. Also, you don’t just slap windows anywhere on the exterior wall, but plan with regard to aesthetics and usable space.