As for what I hear, there are times when I take liberties with my pieces and add a few extra instruments here or there. If I hear something in my head that is for piccolo, 4 trombones, or bass clarinet then I’ll throw it in even though it breaks from my traditional setup. Rarely do I split horn sections indivisi though.
But it’s mainly the same ensemble on every song. The reason for this is because I hope to tour one day, or at least one can hope and fight. So I would need the ensemble (and therefore my arrangements) to be for the same exact group. It would get sloppy if I did string quartet one week and brass quintet the next.
I mainly hear different melodies or embellishments rather than the size of the orchestra, though. For example, Nine Inch Nails ‘Closer’ which comes out this week, is originally a 6 minute song, but I rearranged it to be 5 minutes. This is partly due to licensing issues (songs over 5 minutes cost more for me), but it’s also because I heard the song slightly different. For an orchestra piece, I didn’t like the long drum intro or the stretched out middle section. I’m pretty thrilled with how it came out. If you compare the original to my piece then you’ll hear how I think (I equate my process to a film editor, if anything).
My ensemble size is based off of Beethoven’s traditional orchestra (flutes, oboes, clarinets, bassoon, french horn, trumpet, trombone, timpani, crash, vibraphone, xylophone, piano, roto toms, violin 1, violin 2, viola, cello, bass)