Have you read the very useful information the AB has on the website? It's
here.In general terms you don't need biographical information at LRSM except where it relates to the piece in question. So, for example, it's relevant to know that composer X wrote piece Y just after his wife died, or that composer P intended piece Q as a wedding present for his future bride. You don't have to go into the full details of the composer's life otherwise.
It isn't necessary to divide your words equally between the pieces, by the way. You may need to take more words to describe a sonata than a single piece.
Remember that you are writing for a musically-literate audience (albeit an imaginary one!) who will want to be told something they don't know rather than general information they already do.
Good luck!