It works for me fine on linux and win, even if I set the dpi to 120 (on win).
The size of the Codecompletion Listbox gets calculated by (wx)scintilla and can not be set by the user (at least not without changing the sources and recompilation of C::B).
Which font do you use for C::B ("Settings -> Editor... -> Font -> Choose...") ?
Which version of C::B is installed ?
If self-compiled, against which version of wxWidgets ?
I don't have an idea what happened, but I want to try to reproduce it, so I think as many information as possible is needed.