LyX ⇒ Size of section modifies size of index and gap end of page

[f15]

Hi. This is my first post

I have 2 problems about format (exported to PDF). I have written my thesis (physics) with Lyx 2.03. Document class (book)

1 -
a) I don't know how to modify the size of chapter/section... ,this is: Chapter 1 "Equations of..." .I can modify the size of "Equations of..." but not the size of Chapter 1. The same with sections and subsetions (which have basic text size)
b) If I modify the size of "Equations of..." in Chapter one, this size modifies the size on the index

2 - When I write a set of equations together and they are at the bottom of the PDF page, automatically move this set to the next page and I have a GAP at the bottom of the previous page. The same happens if I have and equation after text: last line of the text moves to next page.
I can solve this by looking PDF and changing one by one, but there must be an easy way

Thanks