I bought a new coat a couple weeks ago and just realized that the back has a slit of about 14in. Do I need to round the corner, and if yes, is there anything I need to know about how much is considered round? 

If most of the length of the coat is closed and only a minority of its length is split, there is no need to round the corner. If it is mostly split, or even half and half, or if it appears as though it is not mostly closed, you should round off one corner.

Some Poskim say it is enough if it looks round. Other Poskim say that one must make the entire corner of 3×3 Etzbaos (6-6.5cm / 2.4-2.6 in) round. Anything less than this requires Tzitzis.

See picture here.



שו״ע אדה״ז י, טז-יז.

שיעור העיגול – ראה א״א מבוטשאטש י, להקל. אבל ראה מור וקציעה שם. משנ״ב שם ט בבה״ל ד״ה ולא. קצוה״ש בהוספות לח״א פה, ב בשם מהרי״ל דיסקין. ועוד.