let s:fontsize = 10
function! AdjustFontSize(amount)
  let s:fontsize = s:fontsize+a:amount
  :execute "GuiFont! Hack:h" . s:fontsize
endfunction

noremap <C-ScrollWheelUp> :call AdjustFontSize(1)<CR>
noremap <C-ScrollWheelDown> :call AdjustFontSize(-1)<CR>
inoremap <C-ScrollWheelUp> <Esc>:call AdjustFontSize(1)<CR>a
inoremap <C-ScrollWheelDown> <Esc>:call AdjustFontSize(-1)<CR>a

" In normal mode, pressing numpad's+ increases the font
noremap <kPlus> :call AdjustFontSize(1)<CR>
noremap <kMinus> :call AdjustFontSize(-1)<CR>

" In insert mode, pressing ctrl + numpad's+ increases the font
inoremap <C-kPlus> <Esc>:call AdjustFontSize(1)<CR>a
inoremap <C-kMinus> <Esc>:call AdjustFontSize(-1)<CR>a

GuiFont! Hack:h10

set mouse=a