Follow the steps detailed below. If you don't want to get and compile the bdfresize package yourself, you can skip to step 3 and download a magnified font instead of creating it.
     zcat bdfresize-1.4.tar.Z  | tar xf -
     cd bdfresize-1.4
     xmkmf
     make
     
On Linux you probably have to use:
     make CCOPTIONS='-include /usr/include/bsd/bsd.h' clean all
     
     mkdir $HOME/fonts
     bdfresize -f 2 cursor.bdf | bdftopcf >$HOME/fonts/cursor2.pcf
     mkfontdir $HOME/fonts
     
     
I have prepared some 
cursor fonts
with the following magnification factors: 1.5, 2, 2.5, 3, 4, 5,
6, 7, 8 and 16. You can download one of them an copy it to
$HOME/fonts if you don't want to use bdfresize.
.xinitrc or .xsession file: before
any X client (that uses cursors) is started the following
commands must  be executed: 
     xset +fp $HOME/fonts
     xsetroot -cursor_name X_cursor
     
That's it—now all mouse cursors should have doubled in size.