How can I get a half point size in LaTeX? Many books use 10.5 pt. Setting point size to 10.5 seems to have no effect. Is there a package that can do that?
Historically speaking fonts used by LaTeX came only in well-defined sizes and the intermediate sizes were not available or only if you compiled them yourself. For example Computer Modern Roman existed in 5,6,7,8,9,10,11,12,14.4 and a few others.
LaTeX knows what sizes are available for a certain font family, and if you you are using it out of the box it uses Computer Modern and the half points sizes aren't available. If use use Type 1 fonts then typically all sizes can be used and something like
The underlying mechanics are a little complicated: The LaTeX Companion devotes a whole chapter on how this all works and how NFSS (New Font Selection Scheme) is set up for new fonts. You can also look at fntguide.pdf that explains the technique behind it.