io per ora non ho tempo di provare, ma potrebbe essere interessante:
% \subsection{Big Integrals}
% \LaTeX\ does not treat integral signs as variable sized symbols,
% in the way that it treats delimiters such as brackets. In common
% with summation signs and a few other operators, they come in
% just two fixed sizes, a small version for inline mathematics, and a
% large version used in displays. In fact by default \LaTeX\ always
% uses the same two sizes (from the 10\,pt math extension font) even if
% the document class has been specified with a size option such as
% \texttt{12pt}, or if a size command such as |\large| has been used.
%
% The standard \textsf{exscale} package loads the math extension font
% at larger sizes if the current font size is larger than 10\,pt.
%
% The HTML3 math description explicitly states that integral signs
% should be treated like delimiters and stretch if applied to a large
% math expression. By default this package ignores this advice and
% treats integral signs in the standard way, however an option
% \texttt{bigint} does cause integral signs to `stretch’ (or at least
% be taken from a suitably large font). The standard Computer Modern
% fonts use a very `sloped’ integral which means that they are
% not really suitable for being stretched. Some other math fonts, for
% instance Lucida, have more vertical integral signs, and one could
% imagine in those cases making an integral sign with a `repeatable’
% vertical middle section so that it could grow to an arbitrary size, in
% the way that brackets grow.ciao
f
Ho trovato il documento a cui fai riferimento nel messaggio (“The typehtml package”), però pare che non riporti modi per avere un integrale che assuma automaticamente la dimensione dell’integrando. Quello che forse ti ha tratto in inganno è la parte finale del testo da te riportato, in cui si parla di stretched integrals: purtroppo questo si riferiva ad un modo per ottenere formule in su documenti HTML.
Se trovo qualcosa in rete vi faccio sapere.