#Ruby approach 2 rev 1, 76 bytes

->s{s=s.chars*' '
0.upto(w=s.size**0.5-1){|i|puts' '*(w-i)+s[i*i+i,i*2+2]}}

Optimized using syntax ideas from Vasu Adari's answer, plus a few twists of my own.

#Ruby approach 2 rev 0, 93 bytes

->s{s=s.chars.to_a.join(' ')
w=(s.size**0.5).to_i
w.times{|i|puts' '*(w-i-1)+s[i*i+i,i*2+2]}}

Completely different approach. First we add spaces in between the characters of the input. Then we print out the rows line by line.

#Ruby approach 1, 94 bytes# ->s{n=-1;w=((s.size2)**0.5).to_i (ww).times{|i|print i/w+i%w<w-1?'':s[n+=1],-i%w==1?$/:' '}}

this ended up far longer than anticipated.

w contains the number of printable characters in the bottom row, or equivalently, the number of lines.

Every line contains w whitespace characters (the last of which is the newline) so the idea is to print these whitespace characters and insert the printable characters where necessary.

Ruby approach 2 rev 1, 76 bytes

->s{s=s.chars*' '
0.upto(w=s.size**0.5-1){|i|puts' '*(w-i)+s[i*i+i,i*2+2]}}

Optimized using syntax ideas from Vasu Adari's answer, plus a few twists of my own.

Ruby approach 2 rev 0, 93 bytes

->s{s=s.chars.to_a.join(' ')
w=(s.size**0.5).to_i
w.times{|i|puts' '*(w-i-1)+s[i*i+i,i*2+2]}}

Completely different approach. First we add spaces in between the characters of the input. Then we print out the rows line by line.

Ruby approach 1, 94 bytes

->s{n=-1;w=((s.size*2)**0.5).to_i
(w*w).times{|i|print i/w+i%w<w-1?'':s[n+=1],-i%w==1?$/:' '}}

this ended up far longer than anticipated.

w contains the number of printable characters in the bottom row, or equivalently, the number of lines.

Every line contains w whitespace characters (the last of which is the newline) so the idea is to print these whitespace characters and insert the printable characters where necessary.

#Ruby approach 2, 93 bytes

->s{s=s.chars.to_a.join(' ')
w=(s.size**0.5).to_i
w.times{|i|puts' '*(w-i-1)+s[i*i+i,i*2+2]}}

Completely different approach. First we add spaces in between the characters of the input. Then we print out the rows line by line.

#Ruby approach 1, 94 bytes# ->s{n=-1;w=((s.size2)**0.5).to_i (ww).times{|i|print i/w+i%w<w-1?'':s[n+=1],-i%w==1?$/:' '}}

this ended up far longer than anticipated.

w contains the number of printable characters in the bottom row, or equivalently, the number of lines.

Every line contains w whitespace characters (the last of which is the newline) so the idea is to print these whitespace characters and insert the printable characters where necessary.

#Ruby approach 2 rev 1, 76 bytes

->s{s=s.chars*' '
0.upto(w=s.size**0.5-1){|i|puts' '*(w-i)+s[i*i+i,i*2+2]}}

Optimized using syntax ideas from Vasu Adari's answer, plus a few twists of my own.

#Ruby approach 2 rev 0, 93 bytes

->s{s=s.chars.to_a.join(' ')
w=(s.size**0.5).to_i
w.times{|i|puts' '*(w-i-1)+s[i*i+i,i*2+2]}}

Completely different approach. First we add spaces in between the characters of the input. Then we print out the rows line by line.

#Ruby approach 1, 94 bytes# ->s{n=-1;w=((s.size2)**0.5).to_i (ww).times{|i|print i/w+i%w<w-1?'':s[n+=1],-i%w==1?$/:' '}}

this ended up far longer than anticipated.

w contains the number of printable characters in the bottom row, or equivalently, the number of lines.

Every line contains w whitespace characters (the last of which is the newline) so the idea is to print these whitespace characters and insert the printable characters where necessary.

#Ruby, 94 bytes# ->s{n=-1;w=((s.size2)**0.5).to_i (ww).times{|i|print i/w+i%w<w-1?'':s[n+=1],-i%w==1?$/:' '}}

this ended up far longer than anticipated.

w contains the number of printable characters in the bottom row, or equivalently, the number of lines.

Every line contains w whitespace characters (the last of which is the newline) so the idea is to print these whitespace characters and insert the printable characters where necessary.

#Ruby approach 2, 93 bytes

->s{s=s.chars.to_a.join(' ')
w=(s.size**0.5).to_i
w.times{|i|puts' '*(w-i-1)+s[i*i+i,i*2+2]}}

Completely different approach. First we add spaces in between the characters of the input. Then we print out the rows line by line.

#Ruby approach 1, 94 bytes# ->s{n=-1;w=((s.size2)**0.5).to_i (ww).times{|i|print i/w+i%w<w-1?'':s[n+=1],-i%w==1?$/:' '}}

this ended up far longer than anticipated.

w contains the number of printable characters in the bottom row, or equivalently, the number of lines.

Every line contains w whitespace characters (the last of which is the newline) so the idea is to print these whitespace characters and insert the printable characters where necessary.