/etc/nmh/scan.size is in nmh 1.6-8build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | %4(msg)%<(cur)+%| %>%<{replied}-%?{encrypted}E%| %>\
%02(mon{date})/%02(mday{date})%<{date} %|*%>\
%5(size) \
%<(mymbox{from})%<{to}To:%14(decode(friendly{to}))%>%>\
%<(zero)%17(decode(friendly{from}))%> \
%(decode{subject})%<{body}<<%{body}%>
|