The default reading speed is 200 words per minute. The default speaking speed is 130. Both are estimates. A technical page and a story do not take the same time, and a word count for Chinese, Japanese or Korean is already an approximation.
The character counter uses grapheme clusters for the number people usually mean by “characters”, and it also lists code units and code points. Set the limit that matches the place you are writing for. This site does not ship preset limits for social networks, because those limits change and need a source you have checked.
A goal of 100, 250, 500, 1,000 or a custom number is a progress mark. It does not say that a length will rank anywhere.