Is it a good practice to hide structure definition in C?

Yes, it is a good practice to hide data.

As an alternate to the alloca(foo_size); pattern, one can declare an aligned character array and perform a pointer conversion. The pointer conversion is not fully portable, though. The character array needs to be a VLA, if the size is defined by a variable and not a compile-time constant:

extern size_t size;

struct sfoo;

#include <stddef.h>

int main(void) {
  unsigned char _Alignas (max_align_t) cptr[size];
  // or unsigned char _Alignas (_Complex  long double) cptr[size];  // some widest type
  struct sfoo *sfooptr = (struct sfoo *) cptr;
  ...

If VLAs are not desired or available, declare the size as a constant (#define foo_N 100) that is guaranteed to be at least as much as needed.

Function bar invokes undefined behavior: the structure pointed to by foo is uninitialized.

If you are going to hide the structure details, provide a foo_create() that allocates one and initializes it and foo_finalize that releases any resources and frees it.

What you are proposing could be made to work, but is error prone and is not a general solution.