I would like to place the elevation-number of a point in a different front size and centred under its name:
Is that possible?
(That is my actual labeling:
label || '\n' || elevation)
Ignoring font size for the minute, you could achieve your first option by centering the label above the point, in QGIS 1.9 you have the option to center the text under the multiple lines section. If you have included a newline character in your expression you can leave the 'Wrap on character' box empty (or I put a slash in my expression instead of the '\n' and set that as the wrap-character - the wrap-character isn't shown). This will set your text centered as in your second option. For something like your first option, you could set the line height option to 2 or 3.
However, QGIS advanced labeling only has a single option for font size. So, to achieve the different font sizes, I think you will have to add your layer a second time and set the elevation font and position in there.