I have defined centuries number for my book. The editor wants me to get the roman number of the century in a smaller size, I've difined the century as follow (it is in french) :
The font commands like \small, \tiny, \footnotesize are not commands with arguments. They apply within the entire scope containing them. If you want to limit their application, you either need to use the environment \begin{small} ... \end{small}, or else put the \small command inside an extra pair of braces, like so: {\small Text}.
So in your case, maybe you want something like this:
(I couldn't even the command you gave, however, since I don't know how "\up" is defined, and since you had \} instead of } at the end, the number of braces was mismatched. A true minimal working example is always most helpful.)
instead of using specific font size commands you could consider to change the size relative to the current font size. The relsize package has been made for this purpose.
Just think of using century numbers also in section or chapter headings where the surrounding text is bigger.