Adding to Nathan's (accepted) answer:
I wanted to cycle through the list of resolutions but didnt see anything for it:
function vncNextRes()
{
xrandr -s $(($(xrandr | grep '^*'|sed 's@^\*\([0-9]*\).*$@\1@')+1)) > /dev/null 2>&1 || \
xrandr -s 0
}
It gets the current index, steps to the next one and cycles back to 0 on error (i.e. end)
EDIT
Modified to match a later version of xrandr ("*" is on end of line and no leading resolution identifier).
function vncNextRes()
{
xrandr -s $(($(xrandr 2>/dev/null | grep -n '\* *$'| sed 's@:.*@@')-2)) || \
xrandr -s 0
}