Here size.find_all('li', class_='col-xs-6 col-sm-8 col-md-6 col-lg-4')
return a list. So you need to iterate it to get the size.
import requests
r = requests.get("https://www.hypedc.com/nz/nike-air-force-1-07-black-black-cw2288-001")
soup = BeautifulSoup(r.content, 'lxml')
size = soup.find('div', id="size-selector-tab-mobile-0")
size = size.find_all('li') # return list
for t in size:
print(t.a.text.strip())
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