Does anyone know how many characters can be placed in an hash table data?

Basicly, I want to make a table that holds comments.
So: <table> <id> <comment>

But before storing this comment, I want to make sure, that it doens't exceed the maximum amount of characters the table can hold.
if ($len(%comment) < x) {

So, what's the max?
Or, what's a safe length to use?

Thanks smile