Why can't I change font size for each responsive size?

Totally confused, both of us. You don't seem to understand the difference between Breakpoint Views workflow and Fluid Boxes at all. One of the big advantages of Fluid Boxes is that fontsize is adapting automatically to all screen sizes until the minimum font size is reached. In that case when the text doesn't fit anymore in the text container, the expand text icon will appear. Setting fontsize for specific screens is however necessary with Breakpoint Views workflows, because there the font size remains fixed between the breakpoints. You have to set up the text style to have a different font size for each breakpoint to cope with that? I explained this in my comparision:

Fluid Boxes Or Breakpoint Views? - Captivate blog

I unmarked your question as correct, because  what you propose makes no sense. If you unlock items or fluid boxes from the parent fluid box, you lose the advantage of fluid boxes where everything is rearranged, based on the settings for the parent fluid box, when the screen is getting too small for the ooriginal layout. It is like taking an engine out of a car, and ranting why it will not drive. You answer set as correct would confuse all CP-users looking at this thread. I rarely use my moderator powers but felt compelled to do so in this case.