|VisualEditor/VisualEditor : master||Provide tools to make text bigger or smaller|
Correct (though we didn't invent it).
Wikitext editor's toolbar has buttons for this:
I'll note that there's a task to remove them: T40487. You could probably file a similar one for VisualEditor. But as long as one has them, the other should too.
Actually, VE will have one until it's banned in wikitext, not just "not easily accessible".
At the bare minimum you'll need to run a three-month-long community RfC on meta with convincing arguments that explain as to why they all need to replace <big> with <span style="font-size: bigger;"> and so on. There are, from a quick search, over 70k pages with at least one of these on the English Wikipedia alone; the transition will be quite a lot of work.
It's not remotely worth it to spend that much in terms of WMF resources. If you want to try, you're welcome, but I seriously doubt you'll get sufficient consensus (and even then it'd probably need to be a configuration option so that third party wikis can address the breaking change in slower time).
Not of this is germane to this ticket, however.