Toggle tab size in .vimrc with <f3>. If you start in some other mode, you'll probably want to set the customtabsize state of your TabToggle() function to that mode.

.vimrc

" Apparently some vim environments require colons before commands in
" .vimrc and others don't. This file has been primarily tested in 
" Ubuntu 12.04, 13.10, and 14.04 environments, where a : is required
" on each line before a command. According to one stackexchange topic
" it is best to leave the leading colons because systems that do not 
" require them can still execute them.
" Author - Trent Robbins

" Turn arrow keys off:
" To cycle history in `ed` mode, use C-n and C-p (although up and down might work fine)
inoremap  <Up>     <NOP>
inoremap  <Down>   <NOP>
inoremap  <Left>   <NOP>
inoremap  <Right>  <NOP>
noremap   <Up>     <NOP>
noremap   <Down>   <NOP>
noremap   <Left>   <NOP>
noremap   <Right>  <NOP>

:set background=dark

:set scrolloff=5

" set backspace for mac delete key (otherwise it doesn't work)
" :set backspace=indent,eol,start

" syntax by default
:syntax enable

" inserts spaces instead of a tab
:set expandtab

"maintain tabbing increments when backspacing tabs (speed things up)
:set smarttab

"show the commands which you enter in the bottom right corner
:set showcmd

" set spaces used for indentenation
:set shiftwidth=4
:set tabstop=4
" does a tabstop of 4 during edit commands
:set softtabstop=4

" Turn on line numbers:
:set number

" indent to previous line automatically
:filetype plugin indent on

" allow input toggle to paste mode with F2
:set pastetoggle=<F2>

" customtabsize's state informs the function what state you are currently in
" if the default settings change, then this variable would need to be changed too
" try this... :let customtabsize = 0
:let g:customtabsize=1
" allow toggle between 2/4 spaces.
function TabToggle()
  if g:customtabsize
    :set shiftwidth=2
    :set tabstop=2
    :set softtabstop=2
    :let g:customtabsize=0
  else
    :set shiftwidth=4
    :set tabstop=4
    :set softtabstop=4
    :let g:customtabsize=1
  endif
endfunction

" TabToggle dumps you at the top of the document so you need to mark your
" place... set scrolloff=5 or higher to have a better time finding the cursor
:nmap <F3> mz:execute TabToggle()<cr>'z

" Quickly quit without saving with QQ
:nmap QQ :q!<cr>

" Maintain undo history between sessions
:set undofile 
:set undodir=~/.vim

:set undolevels=1000 " This many undos are saved.
:set undoreload=10000 " This saves 10000 lines of undos, which is the default

:set foldmethod=indent
:set foldlevel=99
:nnoremap <Space> za

" Keep visual block after changing indent
:vmap > >gv
:vmap < <gv

" Highlight things that we find with the search
:set hlsearch
" <Ctrl-l> redraws the screen and removes any search highlighting.
:nnoremap <silent> <C-l> :nohl<CR><C-l>

" Allow .MD files to automagically use markdown syntax
:au BufNewFile,BufReadPost *.MD set filetype=markdown
:au BufRead,BufNewFile *.md set filetype=markdown

" Set Cursor row and column colors
:highlight CursorLine ctermbg=Black cterm=NONE
:highlight CursorColumn ctermbg=Black cterm=NONE
" :highlight CursorColumn ctermbg=LightMagenta cterm=NONE

" Set Cursor row and column to on.
augroup CursorLine
  au!
  au VimEnter,WinEnter,BufWinEnter * setlocal cursorline cursorcolumn
  au WinLeave * setlocal nocursorline nocursorcolumn
augroup END