Though never explicitly mentioned, we are lead to believe that Madame Maxine is in fact a half-giant.
This is a tricky one since all we have (from the books) is Harry's viewpoint; and Harry only hears allusions to this issue, and mostly in passing.
But from what he hears, he definitely concludes that she must be half-giant:
“I don’t know who Maxime thinks she’s kidding,” Harry said,
watching Madame Maxime sitting alone at the judges’ table, looking
very somber. “If Hagrid’s half-giant, she definitely is. Big bones . . .
the only thing that’s got bigger bones than her is a dinosaur.”
-Harry Potter and the Goblet of Fire, Chapter Twenty-Three (The Yule Ball).
Given the evidence, and Madame Maxine's reaction to Hagrid bringing the topic up, we can also safely come to the same conclusion:
“Eh?” said Hagrid blankly. “No, don’ go! I’ve — I’ve never met
another one before!”
“Anuzzer what, precisely?” said Madame Maxime, her tone icy.
Harry could have told Hagrid it was best not to answer; he stood
there in the shadows gritting his teeth, hoping against hope he
wouldn’t — but it was no good.
“Another half-giant, o’ course!” said Hagrid.
“ ’Ow dare you!” shrieked Madame Maxime. Her voice exploded through the peaceful night air like a foghorn; behind him, Harry heard Fleur and Roger fall out of their rosebush. “I ’ave nevair been
more insulted in my life! ’Alf-giant? Moi? I ’ave — I ’ave big bones!”
-Harry Potter and the Goblet of Fire, Chapter Twenty-Three (The Yule Ball).