Is it necessary to keep untrimmed beard or one can trim it to manageable size?
The Ahadith will give you the answer in this:
Ibn Umar said: The Apostle of Allah (may peace be upon him) said: Trim closely the moustache, and let the beard grow.
(Sahih Muslim)
So you should trim the moustache and let the beard grow. I say go and check what the scholars have said, and see if there are any past rulings on allowing the trimming of the beard. This is what I know. There are many other Ahadith on this matter from other Sahaba but I gave you the shortest one for clarity and simplicity.