As far as you know ,what is the size of the smallest flute ? ( You might have come across different sized flutes)
Once upon a time, the term flute (flauto) properly referred to what is today called a recorder (also known as a type of fipple flute). If you'll accept a recorder as a possible answer, the smallest recorder I know of is the Garklein (German for "quite small"), which is only 16-18 cm.
If you are not interested in recorders and are looking just for transverse flutes, the smallest common flutes available are either the piccolo or the fife with both being about 30cm in length.
There are a number of other transverse flutes including the bansuri, venu, dizi, fue, daegeum, junggeum and sogeum but given that these are traditional flutes not from the west, information online on their dimensions seems to be lacking. However, from looking at pictures none of these appear to be noticeably shorter than the piccolo or fife.
Given that a transverse flute is essentially a pipe with holes drilled into it, I can imagine many people have made smaller transverse flutes for the fun of it. Here's an example, it sounds terrible but with some effort and a slightly longer pipe something workable surely could be created.