When stubedit is used to set the bufsize=64k,
_go32_info_block.size_of_transfer_buffer is set to 2160.

Tested with:
#include <go32.h>

int main() {
  printf("size of tb %d\n", _go32_info_block.size_of_transfer_buffer);
  exit(0);
}

stubedit a.exe bufsize=64k

C:\>a
size of tb 2160

This does not come up with bufsize 63k or any lower value k I can determine.
It does not matter if stubedit is in interactive mode or not.

Solved in v2.02.  64KB-large transfer buffer is impossible, so
v2.02 limits it instead of letting it wrap around...