Pages: can I change the document font size but retain the italics?
I'm not quite sure what you're doing to affect the italics. Changing one aspect should not change any other.
Select all text. Multi-style selections will show as Multiple in the style box. In the Text tab click the size button… all else is retained except the font size.
Window shrunk to only show relevant area…

If your text already has multiple font sizes, meaning the point size box is greyed out, then use  Cmd ⌘   and  +  or  -  which will change all by one point at a time, maintaining relative sizes.
If you use that box in this instance, all type will be set to the same font size, breaking the relationship.