Originally Posted by Guessed

Once a character style has had a size or typeface chosen it can be ridiculously hard to remove that. The clue is to pretty much start again. Because it is a built-in style you do this by:
Select a paragraph and press Ctrl-Q and Ctrl-Space to make sure there is absolutely no 'local' formatting in that selection.
Then, show your list of styles and Right Click on the style 'Hyperlink' and choose 'Update Hyperlink to Match Selection'. This will effectively remove all attributes from the character style.
Now, modify the style 'Hyperlink' to change its colour and add an underline. DON'T touch the size or typeface settings.
That is it. Now, anytime you add a hyperlink it will automatically have the Hyperlink character style applied which should just change colour and underline

Originally Posted by Guessed

without changing size