The string length parameters would be used nowhere in gspell, currently. They would always be -1. But for some widgets it's probably possible to traverse the text without extracting each word… Doing lots of small memory allocations can be bad for performance, and doing stack allocation can be cumbersome if a word is longer than the buffer (and it still needs a copy).

So in the end, it's probably better to add string length parameters. Passing -1 is easy.

Only the GspellChecker class is concerned. For the signals, it's more convenient to not have the length parameters. The most important is for gspell_checker_check_word(), since it's called for every word, whereas the other functions are called less often.