I can just drag it. But when I drag it I don't know the height of it in pixels. How could I adjust it accurately so that i know the height?
I know this is an old question but I think I have a good answer for it:
the attribute name you are looking for is "thickness"
There you can specify for example
Log out and log in back again to see changes.
If you have changed the resolution, there will be a section specifying the panel values for each used resolution, so to see changes you will have to change the one that corresponds to your current resolution.
Just use KRuler - it looks like this: