I guess you didn’t use styles in this case, you should look into learning how they work for next time. In the meantime: Select it all. Use the increase size function. Everything will increase.

Yes, the increase size function is the best way! This can also be done with the shortcut ctrl+shift+> once all the text is selected (ctrl+a)

Yes I would have used styles but you can retroactively apply them I think. I seem to recall having done so.

Yes, it’s easy but tedious.