Roughly the same size as if the street had 2 car lanes on each side. In fact this is what I've seen living here in Amsterdam for a few years, every once in a while they remove a lane or two from some street and beef up these security features as well as add more pedestrian space.

It's cheaper to maintain extra fat sidewalks and stuff than 2 more lanes of asfalct also.

Even better, the gemeente is actively converting streets into fietsstraat. It is amazing and I love it. It makes my commute through the city so much faster and less stressful. When they did the knip experiment on that big through-road near Waterlooplein and there was no car traffic, it was also fantastic. At that time, I was commuting that direction and it was wild how quiet that part of the city became. Cars really are a terrible nuisance and do not belong in the city.

Also, very often it doesn't reduce flow even for cars. There are tons of times when you remove lanes and it improves are keeps flow constant.

4 lane roads are the worst, you can get the same effect with a 2 lane with a turn.

It really depends on how many intersection you have, having a single lane that only branches to 2 in front of an intersection can be more efficient, then constantly 2 lanes.

The US style of many lanes, many intersections, is horrible from safety and a flow perspective.