Ah, yeah. I think the only thing that's going to work is to just paste the
Excel "table" itself. So you'd have to choose "Excel object" in Edit|Paste
Special. Or choose "Excel table" from the paste options icon thing that
appears right after you paste. To do this, you would not paste into the
table cells on the slide, you'd just paste directly on the slide (in other
words, don't select table cells before you paste).

But, I know where the table font settings come from. It's not
straightforward at all.

If you copy in Excel and select the table cells in PPT and choose Edit|Paste
Special|Formatted Text, here's where the settings come from:

Font Size = the next size down (in the font dropdown on the toolbar) from
the size set in the text placeholder on the slide master
Font Face = the font face used in the Excel cells
Font Color = the font color used in the Excel cells
Alignment = the alignment set on the text placeholder on the slide master

So, if you create an additional slide master with a placeholder that uses
Arial 20 pt and is right-aligned, and apply that slide master to your table
slides, you should get the results you want when you paste.

If you choose Edit|Paste Special|Unformatted Text, the table gets all its
stuff -- size, font face, color and alignment -- from the text placeholder
on the slide master. The size is the same as with formatted text -- it's the
next size down from the size in the text placeholder on the slide master.