there are a couple of variables, NBUF and BUFPAGES.  they are documented
in options(4) (in netbsd-current):

     options NBUF=value

     options BUFPAGES=value
     These option set the number of pages available for the buffer cache.
     Their default value is a machine depedant value, often calculated as be-
     tween 5% and 10% of total available RAM.

so, on the i386 (that has a 4096 byte page size), if you want 10M of
memory for disk cache, set BUFPAGES to 10M/4K (2560).  NBUF will
typically be set to the same as BUFPAGES.