C (gcc), 354 bytes
h,i,j,k,l,m;g,v,w,d;f(char*s){for(i=l=0;s[i];l+=!isspace(s[i++]));d=g=l;for(i=0;i++<l;)if(h=i*i-l)for(j=0;j<i-++j;h>0&h%k<1&k<=m&m<i&m+~k&&m-k<g|m==k+g&(d-2*v)*(d-2*w)>h?w=(i-m)/2,v=j,g=m-k,d=i:j)k=i-2*j,m=h/k;else g=0,v=w=d=i;if(g-l)for(i=j=1;*s;j+=!i++,k=0,i%=d+1)for(putchar(i?v>=j|j>d-v|w>=i|i>d-w?k=1,*s:32:10);k;k*=isspace(*++s));else puts(":~(");}
att
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