Cheddar, 27 bytes (non-competing)

s->s.chunk(s.len**.5).vfuse

I added the .chunk function a while ago but I removed it in the transition to the new stdlib format and forgot to re-add it. Cheddar has a dedicated sqrt operator but **.5 is shorter

Try it online!

Explanation

s ->              // Function with argument s
    s.chunk(      // Chunk it into pieces of size...
      s.len ** .5 // Square root of length.
    ).vfuse       // Vertical-fuse. Join on newlines

Cheddar, 27 bytes

s->s.chunk(s.len**.5).vfuse

I added the .chunk function a while ago but I removed it in the transition to the new stdlib format and forgot to re-add it. Cheddar has a dedicated sqrt operator but **.5 is shorter

Try it online!

Explanation

s ->              // Function with argument s
    s.chunk(      // Chunk it into pieces of size...
      s.len ** .5 // Square root of length.
    ).vfuse       // Vertical-fuse. Join on newlines

Cheddar, 27 bytes (non-competing)

s->s.chunk(s.len**.5).vfuse

I added the .chunk function a while ago but I removed it in the transition to the new stdlib format and forgot to re-add it. Cheddar has a dedicated sqrt operator but **.5 is shorter

Explanation

s ->              // Function with argument s
    s.chunk(      // Chunk it into pieces of size...
      s.len ** .5 // Square root of length.
    ).vfuse       // Vertical-fuse. Join on newlines

Cheddar, 27 bytes (non-competing)

s->s.chunk(s.len**.5).vfuse

I added the .chunk function a while ago but I removed it in the transition to the new stdlib format and forgot to re-add it. Cheddar has a dedicated sqrt operator but **.5 is shorter

Try it online!

Explanation

s ->              // Function with argument s
    s.chunk(      // Chunk it into pieces of size...
      s.len ** .5 // Square root of length.
    ).vfuse       // Vertical-fuse. Join on newlines

Cheddar, 27 bytes (non-competing)

s->s.chunk(s.len**.5).vfuse

I added the .chunk function a while ago but I removed it in the transition to the new stdlib format and forgot to re-add it. Cheddar has a dedicated sqrt operator but **.5 is shorter

I don't think this needs explanation but if you'd like one I can add one.

Cheddar, 27 bytes (non-competing)

s->s.chunk(s.len**.5).vfuse

I added the .chunk function a while ago but I removed it in the transition to the new stdlib format and forgot to re-add it. Cheddar has a dedicated sqrt operator but **.5 is shorter

Explanation

s ->              // Function with argument s
    s.chunk(      // Chunk it into pieces of size...
      s.len ** .5 // Square root of length.
    ).vfuse       // Vertical-fuse. Join on newlines