All is in the title, we can choose difficulty (make me laugh) but not the size ? I don't want to lose my time on very huge outpost and would like to have the choice about that
There is a very simple issue with this notion. No one would choose to play anything but small. Why would you? The extra tombs aren't worth the extra time. Therefore anyone who built an outpost that is any size but small would never get raids. If humans have proved one thing, it's that the vast majority will take the path of least resistance. The fact that normal raids get large amount more raids than higher difficulty despite smaller rewards prove it.
U know spend 10/15 minutes on huge long and boring outpost and finish on a kill box, I'm tired about it, so yes I prefer small size on medium dificulty and many player prefer a fast run because they don't have enought time to play.
You always have the option of using the suit advisor boost to see how long the HRV trail is on a map before entering. If you feel the trail is excessively long then load the map and abandon it to get a new one on your expedition. A larger map does not necessarily equal a longer time to beat. Likewise, a small map doesn't necessarily equal quick.
I can build a big old winding tower on a small map that would take 10+ minutes for most players to get through or a direct to gen mat, easily marked trail on a large map that takes 30 seconds to beat. If builders are forced to only build on small maps (and lets be clear here, this would force anyone that wants raids to only build small maps) then builders who want to build a more immersive experience that takes a few minutes to get through, still would make those maps.
The end result would be a worse game overall. If you just want quick, easy wins then use the suit advisor boost, stick to normal difficulty and abandon anything 300+ meters in length. Regardless of the map size, it shouldn't take more than like 2 minutes or so to run through. I'm not part of the dev team so I can't speak in absolutes but I can virtually guarantee that they will never add a map size filter to expeditions.