Um caso sério de RTFM, por minha conta. As poucas linhas superiores explicam como editá-lo.
" Each "set" line shows the current value of an option (on the left).
" Hit <CR> on a "set" line to execute it.
" A boolean option will be toggled.
" For other options you can edit the value.
" Hit <CR> on a help line to open a help window on this option.
" Hit <CR> on an index line to jump there.
" Hit <Space> on a "set" line to refresh it.
Em seguida, pressionei Enter e minha configuração entrou em vigor imediatamente
on line 155
lines number of lines in the display
set lines=50