Somehow I don't find a way of changing the font size of a specific string of text.I know how to do it globally or temporarily for the whole buffer.Changing the default font is not the issue here.Sometimes I use math symbols like ,  or  and want to change these to a bigger font. I'd need a simple way of doing this. Alternatives to make these symbols more visible are welcome too.
Emacs change font of a single word (or sentence), not of the whole buffer
emacs;fonts
What you are looking for is an Emacs face that uses larger size characters. See the face attribute :height, for instance, in the Elisp manual, node Face Attributes.You can apply a face to any piece of text (e.g., a word or expression).  You can do this by program or manually.  If by program, it can be done using font-lock-keywords (automatic syntax highlighting) or using any of various text highlighting functions.Such highlighting can be effected using the face text property or the face overlay property; that is,  it can be attached to buffer text (or to a string), on the one hand, or overlaid on top of the buffer, on the other hand.See also:EmacsWiki - Faces categoryEmacsWiki - Font-LockEmacsWiki - HighlightingEmacs manual, node Enriched Text