#Java 1.7, 110 bytes

void f(String s){for(int i=-1,k=(int)Math.sqrt(s.length());++i<k;)System.out.println(s.substring(i*k,i*k+k));}

Try it! (Ideone)

I tried another approach with a function returning the result as a string, but just having to declare the string and the return statement is already more expensive (byte-count-wise) than the print statement.

Gotta love Java's verbosity... :)

Java 1.7, 110 bytes

void f(String s){for(int i=-1,k=(int)Math.sqrt(s.length());++i<k;)System.out.println(s.substring(i*k,i*k+k));}

Try it! (Ideone)

I tried another approach with a function returning the result as a string, but just having to declare the string and the return statement is already more expensive (byte-count-wise) than the print statement.

Gotta love Java's verbosity... :)