In V7 reporting, collapsible filters had options to setup their width in px and their height in either px or number of rows. Also there was an option to collapse on mouse leave.
Do these parameters exist as well in new V8 and I just did not find them ?

V8 does not support collapse on mouse leave.
The options about styling are moved into the theme API, see https://github.com/ic3-software/ic3-demo-plugin-theme and https://livedemo.iccube.com/icCube/report/help?ic3topic=api.plugin.Theme&ic3locale=en_US.
E.g., the code below shows where to set the tree item height (using the git example from above).
For the next release, we're going to add an option in the theme for the popup width and height.