Chord Font Size...How do I Increase their size.

You don't have to select them all. As i said, just do one, then press the "Set as style" button (labeled "S") and it automatically applies to present and future chord symbols.

But for the record, to select ask current elements of a given to type, right-click then Select / All Similar Elements.

Format > Style > Text styles > Chord symbols

Well I figured this out myself, though I'm not sure I could describe a systematic way to do it that would work every time. First, I made the Properties panel visible by selecting View->Properties. Then I left-clicked on one particular chord symbol. I was thrilled to see a set of panes appear in the Properties panel, one of which was "Chord Symbol" and another of which was "Text". Under "Text" was a selector for font Style and Size, where I changed the font size. Upon clicking the unlabeled ellipsis to the right of the Size & Style selectors, a two-member list appears. After changing the font size, the second item ("Save as default style for this score") was enabled and I selected it, after which all chord symbols adopted the new size. I've detailed this five-step process in the attached image.

I find it a bit disturbing that doing something as simple as changing chord symbol font size is so non-intuitive that I'd have to write a detailed set of instructions on how to do it, and that I'd have to warn people away from doing what I thought was the most intuitive thing (right-clicking the symbol and selecting Style). The thing that made me gravitate toward MuseScore (and away from proprietary nightmares like Dorico and Sibelius) was its apparent simplicity, back in the days of version 3. It seems that MuseScore is becoming more unusable like these others. It took me well over an hour experimenting, then writing about it, then sleeping on it (not counting the sleeping time here) and then revisiting it the next day just to figure out how to do this simple thing. I'm worried for the future of this product.