I doubt this is zsh's problem but she may have a solution just the 
same.  I do this in my 'i' function:

     width=${=$( stty size )[2]}

... and it works fine, however if I call the function thusly:

    _function_test ()
    {
         local integer i=0
         while read aa; do
             let i++
             eval ${=aa} 2> /dev/null
         done
    }

     $ _function_test <<< "i path"

... 'width' is zero.  I think this is some 'stty' issue, still the 
function will be running in a terminal and the terminal has a width so 
it puzzles me that no width can be reported.