What are the considerations I should make, to determine the top margin in a chapter page and what is the margin I should leave below the chapter title and the text? Do you just add an empty line or deal with style margin?
This is an aesthetics matter. There are as many answers as authors.
Technically, some clarification is needed because words have specific meanings in Writer.
A margin is some space around the page where no text is allowed. Writer explicitly defines margins as no-print areas. The consequence is header and footer are taken from the area defined from the margins. This is the opposite of Word where margins define the text area and header/footer are allocated inside the margins.
If you want to change the margins for the first page of a chapter, you must do this by using a specific page style (which describes the geometric layout of a page) and switch to your standard page style at end of page.
More frequently space around a chapter heading is created by playing with the Indents & Spacing properties of the paragraph style applied to the chapter heading (usually Heading 1).
Never (“never” should be repeated seven times to emphasise the importance of the statement) vertically space with empty paragraphs. Semantically, empty paragraphs contain no information. Your text is intended to give bits of significance to your reader. Therefore, a “void object” is pointless and a document should have no empty paragraphs. In addition, empty paragraphs, though empty, are real paragraphs. As such, they are managed by Writer just like any others. This means they are taken into consideration when laying out text into pages. They interact with the layout algorithm in non trivial ways (apparently from a newbie point of view) and this will play nasty tricks on your back sooner or later. Authors who rely on this typewriter-era trick to synchronise their text with page boundaries are always taken aback by the slightest change in their formatting which ruin their manual synchronisation. Using styles with typographical constraints specification (mainly Text Flow properties) protects you from this mishap.
Modify your question (= edit it with the pencil tool which appears when you click …; don’t use a comment) to describe exactly what you want. Don’t forget to mention your OS name, LO version and save format. Most potentail answers are valid only if document is saved native, i.e. .odt.
Is there a relation to font size used and the margin between Heading 1 and the Body Text below it?
None. You specify spacing below (Heading 1) and spacing above (Body Text) as you like. In the Writer box model for paragraph, spacing around the proper paragraph text has absolutely no relation to font size. Consequently, if you want to create a relationship, you have to compute the “distances” by hand and enter them in the appropriate entry boxes in Indents & Spacing tab.
Don’t hesitate to play with both spacing above and below to create various final distances when paragraph styles are different above and below the inter-paragraph space. Too many people try to go with only one parameter and are a bit disappointed by the result.