:This does work, but the effect is not what you expect. Without <code>yes</code>, you have now set up the default size, but you have not turned paragraph indenting on. You have to execute <code>\setupindenting[yes]</code> before it starts actually indenting your paragraphs.
=== Does $$...$$ work for display math, like in (La)TeX? ===
:No; the spacing above and below the equation will be wrong, and maybe other aspects too. Instead use <cmd>startformula</cmd> and <cmd>stopformula</cmd>.
=== How do I get a numbered equation? ===
:Add <cmd>placeformula</cmd> before the <cmd>startformula</cmd>.