Does crown molding make your rm look larger? Is it hard to put up?

Not necessarily will it make the room look larger but it will make it look more complete. Depending on how many angles you have it can be a bit difficult but worth the effort. You have to make sure you cut the correct angle or have someone that knows how to cope it so it will fit together correctly And have the elegant finish it is meant to have. Good luck with your project and be sure to post pictures of your completed project.

if you have modern mid century or like modern you don't need to. We did the fireplace "breast" wall ( projects out from other walls) because there was water damage at the ceiling. There are miterless crowns now and that's what we used.

It makes a room look finished and could make the walls look higher, depending on the colors you choose to paint. http://www.hgtv.com/design/decorating/design-101/how-to-install-crown-molding

I don't think it makes the room look larger, but it does make it look "finished" and adds interest to even the most boring walls. Installation can be tricky due to the number and types of angle cuts needed to look perfectly installed.