Currently you cannot manage the lineheight in the HTML Editor. An approach that I have found which offers consistency when copy and pasting from MS Word documents is firstly to 'normalise' the line height in the Word document before doing the copy. That is, in MS Word set the lineheight to 1.0 or similar for the whole document, do the copy and then paste into the HTML Editor.
When doing the paste into the HTML Editor, choosing the option to Paste as plain text offers the most consistent result however you do lose formatting that has been copied from the source document.
The idea to have adjustable line heights function in the HTML Editor is a good one and should be submitted to the Product Idea Exchange.
Hope that helps!