They are for different dimensions. „niska” is for height and is the opposite of tall, while „krótka” is for length and is the opposite of long. (Like how in English there are words like “narrow” (opposite of wide) and “thin” (opposite of thick) that mean almost the same thing, but are used for different dimensions.)
So you might say „Niska kobieta pisze krótki artykuł.” (“The short woman is writing a short article.”)
If someone has troubles with remembering the word niski, remember that feminine singular niska, like in this sentence, means the back of the neck (also known as nape) in Finnish. So short neck :) I hope it helps and I hope I didn't confuse anybody :)
Alternatively, "Humans" had a female character called "Niska", who, while not exactly short, was shorter than "Mia".
i don't agreed with this word "short" .for me this suit bether for things then for people .or i agreed with that type of sentence for exemple i have short legs rather then iam short .what do you thing about my arguments . Thanks for your answer .
Well, "short" is used both for people (their height) and for things. This seems weird for a Polish user as well, because for us those are completely different words. "niski" is "short" about height.