$ cat max.txt
abcd:1982:a
efghij:1980:e
klmn:1923:k
opqrst:1982:o

efghij efghij:1980:e
opqrst opqrst:1982:o

#!/bin/bash

#constant
INFILE="max.txt"

#core script
awk ' BEGIN { OFS=FS=":"; cur=max=0; seen=""}
        {
           cur = length($1)
           if(cur > max ){
              seen = $1 " " $0
           }
           else if(cur == max){
              seen = seen "\n"  $1 " " $0
           }
        }
        END { print seen }'  $INFILE
#exit normally
exit 0

opqrst opqrst:1982:o

efghij efghij:1980:e
opqrst opqrst:1982:o

awk ' BEGIN { OFS=FS=":"; cur=max=0; seen=""}
        {
           cur = length($1)
           if (cur > max ) {
              seen = $1 " " $0
              max = cur
           } else if (cur == max) {
              seen = seen "\n"  $1 " " $0
           }
        }
        END { print seen }'  $INFILE

awk '{l=length($1);if(l>=max){a[$1" "$0]=l;max=l}}END{for(i in a)if (a[i]==max)print i}' FS=':' file