This question has probably been answered several times, but I have not yet found a solution. Is it possible to display and change the dimensions of an object or its bbox? Just like you can do in cinema 4D or Maya.

When I import an object, the size is always displayed as 1. However, the lethal word size is often different.
I'm new in Houdini, so sorry if the question sounds stupid.

You can access an objects dimensions with bbox: eg. bbox(0, D_XSIZE).
Position would be centroid: centroid(0, D_X), centroid(0, D_Y) or centroid(0, D_Z) respectively.

Here are more expression functions: http://www.sidefx.com/docs/houdini/expressions/index.html [www.sidefx.com]

Thank you, I test it and it works fine.
Wouldn't it be simpler to be on object level? For example, if I import an FBX object with several sub-objects. How do I know how big it actually is?

You would merge all objects on geo level.

On object level you will only find containers and you would not really know what nodes within are supposed to be measured.