In the Legend, the maximum number of characters you can have per word is 25.
As long as you separate words with a space you can have as many words containing 25 characters as will fit.
There appears to be no reasonable limit to the amount of words you can have, but you can only have one line.
What you type is always centred. If you type too many characters then the beginning and end of the sentence will disappear off the edges of the screen: