I think you'll have to use the method GetRect to know the width of the string, or wait a bit : in the roadmap there is "Add functions in sf::Font / sf::String to get the size of single characters"
In python you could do :
from PySFML import sf
text = 'Seconds before the Earth is demolished to make way for a galactic freeway, Arthur Dent is plucked off the planet by his friend Ford Prefect, a researcher for the revised edition of The Hitchhiker\'s Guide to the Galaxy who, for the last fifteen years, has been posing as an out-of-work actor.'
res = ''
newline = ''
s = sf.String()
for word in text.split(' '):
s.SetText(newline+word)
if s.GetRect().GetWidth() > 400:
res = res + '\n' + newline
newline = word
else:
newline = newline + ' ' + word
res = res[2:] + '\n' + newline
s.SetText(res)
# draw s...