AWK, -F '' -vRS=" " 34 bytes

{NF-1>length(x)&&x=$0}END{print x}

To test:

awk -F "" -vRS=" " '{NF-1>length(x)&&x=$0}END{print x}' <<< "tiny small bigger biggest"

{NF-1>         # length of current word
length(x)      # length of longest
&&x=$0}        # set longest
END{print x}   # print when done