What is the best course?

Is it better to have bigger rooms? I assume there is an increase in output for bigger rooms that makes the superior to have 3 seperate rooms. What is the amount though?

It also seems to be a liablity to have low level dwellers staffing an upgraded room, the incidents there are likely to kill them.

Perhaps it is beneficial to have a junior and senior set of rooms to train lower levels and keep higher levels in.

Every time you connect a room to another it increases the production rate by about 25% above the rooms individually while the time required only is increased by the two rooms combined. It is better to expand your rooms. You also spend less when upgrading them, however beware that upgraded rooms spawn harder rad roaches, mole rats, and tougher to extinguish fires. You should only upgrade your rooms if your dwellers are very high level and have good weapons.

Yes, always expand the most important rooms like Power, Food and Water. Never go into 1 or 2 rooms. With resources You need to have 3 rooms and later fully upgraded to be able to keep your Vault running.

I was going with 2 rooms in training category, so I had like Gym, Class Room, Armory, all with only 2 rooms. At the beggining You don't need to train 6 people at once, 4 is enough.

1 rom I had just for MedBay and RadAway, as with the people with high Intelligence, they were really fast with producing it, without any upgrades.

The rest is up to you, but with most important room (Food, Water, Power) always go with 3 rooms.

It's also best to keep them at the same floor, so when You buy Mr Handy, he can collect all of it from one flor.

Yes, Med Bay and Science Lab improve 1 -> 3 -> 4 a cycle when expanding, so 2 seems to be the sweet spot for expansion for them. Only go for 3 if you want to save caps on upgrading.

Thanks for the info! That helps a lot.