For small sections of text, write a command:
\newcommand*{\TitleFont}{%
\usefont{\encodingdefault}{\rmdefault}{b}{n}%
\fontsize{16}{20}%
\selectfont}
which you can use as
\title{\TitleFont ....}
It's not a biblatex thing, but related to the document class you are using, or a package you are loading. In memoir, e.g., you can write
\renewcommand{\footnotetext}{\normalsize}
and you are good to go. But this won't work across the board. A dirty hack, which cannot be recommended except for the simplest of cases would be to \let footnotesize = normalsize:
\let\origfootnotesize\footnotesize
\let\footnotesize\normalsize
But obviously this will make everything that uses footnotesize be typeset in normalsize, which may cause other, bigger problems. (The \origfootnotesize will let you manually call the original footnotesize declaration if you need it for some reason.)
\documentclass[12pt]{article}
\usepackage{lipsum}
\let\origfootnotesize\footnotesize
\let\footnotesize\normalsize
\begin{document}
XXX%
\footnote{\lipsum[1]}%
YYY%
\footnote{\origfootnotesize\lipsum[1]}%
\end{document}
For the article class you could redefine the definition of @footnotetext (from article.cls) in this way:
\makeatletter
\long\def\@footnotetext#1{\insert\footins{%
% \reset@font\footnotesize% <-- original
\reset@font\normalsize% <-- change
\interlinepenalty\interfootnotelinepenalty
\splittopskip\footnotesep
\splitmaxdepth \dp\strutbox \floatingpenalty \@MM
\hsize\columnwidth \@parboxrestore
\protected@edef\@currentlabel{%
\csname p@footnote\endcsname\@thefnmark
}%
\color@begingroup
\@makefntext{%
\rule\z@\footnotesep\ignorespaces#1\@finalstrut\strutbox}%
\color@endgroup}}%
\makeatother
And this should give you what you want. The definitions for report.cls and book.cls are likely the same, but I haven't looked.
Best Answer
Are you looking to change some definition in your preamble? If you just need it for a specific set of paragraphs, you can just use
\smallor a similar command like David suggests (with or without{}).For what the actual sizes are, have a look at this good answer here. I believe a section header is normally
\Large, a subsection is normally\large, and a subsubsection is normally\normalsizeso that might give you some perspective. A command like\sectionfont{\large}or\subsectionfont{\normalsize}should change the size of all the applicable headers after it.For a more in depth answer, check out what Gonzalo had to say.