Alfa Beisa -nun pshuta shall be like zayin but longer,do not shorten less than 4 kulmus including head
Alfa Baisa 2 nun pshuta shall be like zayin however length shall be 4 kulmus including head so it can be bent into nun kfufa
Darkei Moshe siman 36 quotes Boruch Sheamar "nun pshuta shall be 4 kulmus including rosh.
Mikdash Me'at" os nun: (4)quotes Darkei Moshe and stresses 3 kulmus for regel
Mikdash Me'at" os nun: (5)quotes Darkei Moshe in name of Mordchai IF NUN SOFIT WAS WRITTEN LESS THAN ABOVE SHIUR IT IS STILL KOSHER ,if in doubt then show to tinok
Ksov Lechaim-Shmulevitzer http://hebrewbooks.org/pdfpager.aspx?req=32842&st=&pgnum=24
has no mekor but writes it should have four and a half kulmusim (now remember he is not a posek actually according to many he misquotes the boruch sheomar many times.)
Rabbi Silberger pictures nun sofit with 3 and a half kulmusim and quotes above ksov lechaim and meir eini sofrim http://hebrewbooks.org/pdfpager.aspx?req=40429&st=&pgnum=47&hilite= that boruch sheamar os kuf and darkei moshe disagree.-Mikdash me'at in letter kuf however explains baruch sheamar that he agrees to darkei moshe that 3 kulmus is the shiur and a bit less is also kosher and as regel of kuf 3.5 kulmus including head is called "as nun sofit".
(I don't really have a complaint to Rabbi Silibiger since a longer nun sofit is fine and actually should be longer next to an ayin or peh)
NOW HERE IS A QUESTION TO ELI'S DESIGNATING A GROOMED TINOK(which I assume a new tinok has to be groomed every year)
1)When teaching nun sofit how long is the regel on the picture that you teach-if it is longer then 3 kulmusim then your tinok would not be acceptable.
one more point when the braysa warns not to write zanin nunin to which nun does the braysa refer
in my opinion the braysa is refering to less then 3 kulmus (same as yudin vavin -for sure the braysa does not have to warn us not to misspell a word)