The מנחת יצחק's answer

 

This same שופר is a שופר כשירה לכתחילה for ראובן and a  שופר פסולה אפילו בדיעבד for שמעון. (not stolen etc.) The ביאור הלכה תקפו:ט writes the minimum שיעור שופר is a טפח and according to the  ריטב"א we don't measure this טפח as an average טפח but it is measured according to each individual's fist. Hence if ראובן has a small fist and שמעון has a large fist there is a possibility that only ראובן will be יוצא with a  שופר that's big enough for him but too small for שמעון. Harav Hagaon R Dov Landau Shlita (R.Y. Slabodka) claims, that even the ריטב"א will hold the שיעור שופר is an average טפח but there is an additional הלכה that both ends of the שופר must be visible while blowing. According to R.D.L. If the large fisted שמעון won't hold the שופר with his entire fist but use only a few fingers and both ends of the שופר are visible even the ריטב"א will agree he will be יוצא תקיעת שופר