I doubt that it can count characters and also give you a good response.
If your response is in English, then you can limit somewhat.
Given that the average number of characters in an English word is 4, then you need to have max 400 words to fit in 1600 characters.
OpenAI documentation says that the ratio is 3:4 tokens to words.
It means that the response can be at most 300 tokens.
For safety, I would target a response of 250 tokens, because you might encounter longer words.
Note that the max_tokens parameter includes the prompt as well. So you have to find out how many tokens are in the prompt and add 250 tokens to determine the max_tokens parameter.
You can find the GPT-2 library to count the tokens on GitHub.
PS: the average word length is not 4, but you can find it.