I was wondering and thinking, how big can a stallions #@?!’ (You might know what i mean)
Youtube will answer this so easily that I will not provide a link. 
Penis. The word you are looking for penis.
Now, now - fighting ignorance and all.
The horse penis Wiki says 20 inches but then states that it doubles in length and girth when erect. So, call it 3 1/2 feet. From my experiences in the field, this sounds about right.
Penis, dick, cock, perhaps? Where or when do you live that you have to censor any of those words?
Perhapspizzle is the word he seeks.