Trying to edit the Chapter styles is driving me nuts. What I want is to have a large chapter number, center justified, with the title of the chapter in large font centered below it. I can’t figure out any way of making this happen. When I try to live-edit the chapter heading, hitting enter just creates another chapter, or moves me to the next chapter. How can I easily change/create the chapter heading style that I want?
Hit Shift-Enter after the chapter number. This will insert a soft line break instead of a paragraph break.
Also make sure that the format description for the numbered heading style at Paragraph > Styles > Enumeration says  without any characters following it. Otherwise the number might be off centre.
If you want the font size for the number to be different from the chapter heading then create a text style with the desired font size and apply it to the Numbering text style in the same place (it normally is set to same as paragraph text).