Yup, 255 characters.
As empirically verified while creating a new Steemit post (i.e. this one), and also via the following JavaScript code snippet from the steemit.com GitHub project:
const maxKb = isStory ? 100 : 16
const validate = values => ({
title: isStory && (
!values.title || values.title.trim() === '' ? 'Required' :
values.title.length > 255 ? 'Shorten title' :
null
),
category: isStory && validateCategory(values.category, !isEdit),
body: !values.body ? 'Required' :
values.body.length > maxKb * 1024 ? 'Exceeds maximum length ('+maxKb+'KB)' : null,
})
It also appears from the above code snippet that the maximum story body is 102400 characters (i.e. 100 Kilobytes).