Sounds like the size was explicily set in the character tag instead of AsIs. In that case, you'll shoot yourself in the foot by changing the paratag sizes.
If you've already changed your paragraph tags and see that this has happened, then you've got some cleaning up to do.
What will work is, In one instance of the tagged content, explicitly change the size in the Character Designer to what is required, then click Update All. Then update that character tag definition again to be AsIs for the font size (and most other attributes) and remove overrides. The tagged content should now change if you change the paratag font sizes.
Repeat for other character tags.
However, if you haven't done anyhting yet, then in the Character Designer simply change all relevant character tags to have a size of AsIs and update all to remove overrides. Nothing should visually change at this point. Then, when you change the paratag font size, the character tags will now change size accordingly.