Вот часть minibufexpl.vim
:
" If you would like a vertical explorer you can assign the column
" width (in characters) you want for your explorer window with the
" following .vimrc variable (this was introduced in 6.3.0):
"
" let g:miniBufExplVSplit = 20 " column width in chars
"
" IN HORIZONTAL MODE:
" It is now (as of 6.1.1) possible to set a maximum height for
" the -MiniBufExplorer- window. You can set the max height by
" letting the following variable in your .vimrc:
"
" let g:miniBufExplMaxSize = <max lines: defualt 0>
"
" setting this to 0 will mean the window gets as big as
" needed to fit all your buffers.
Это должно помочь решить вашу проблему.