Arasznyi (“AH-RUS-NYI”) means the “length of an arasz“, which is the distance between the tip of the thumb and the index finger on an average human hand. It is an old Hungarian measurement of length and is a unit of measurement in some Hungarian cookbooks.
For related information, see our guide to Hungarian kitchen measurements.
And if you are really interested in more: kisarasz (small arasz)is the distance between the tip of the thumb and the index finger, while nagyarasz (big/long arasz)is the distance between the tip of the thumb and the tip of the little finger (fingers spread).
Very good explanation. This is called “pedalj” in Serbian!