1) Is it a major sin(gunah-e-kabeera) to cut the beard before a fist length?

1) Is it a major sin(gunah-e-kabeera) to cut the beard before a fist length? 2) I have never cut my beard and letting it grow but in the masjid which is near by my house, the imam cuts his beard before a fist length. I need to know that if I pray salat behind such imam then would my salat be accepted or I need to repeat that salat? 3) If it is necessary to repeat the salat then instead of repeating every salat, can I pray salat with jama't at home with my family members? what is better? jama't at masjid behind that imam or jama't at home? and there is no other masjid near my house. so what should I do? 4) And also please what about jum'a? If I pray jum'a salat behind the same imam then do i have to repeat zuhr salat or jam'a would be accepted? and should I tell other people to do the same? jazakallah for all the advices u gave me before. May ALLAH take lot of deeni work from you. Aameen.