Sorry, I've been going back and forth between sections of the forum and thought I was in the citizenship section when writing my above reply.
So regarding your upcoming green card ... have you applied for it yet? Is your 17-letter first name made up of one word, multiple words with a hyphen, or multiple words with space(s) in between? If it's multiple words with spaces, you can simply treat the first word as your first name, and the rest of it as your middle name(s). Fill out the GC application like that and do the same for everything else (driver's license, SSN, etc.) and you'll have no problems.
But if it's one long word, or two words with a hyphen in between, you would have to request an official name change in court to get the shorter first name onto your GC. You would also need a court-ordered name change to add in the middle name, since it's not on your birth certificate or passport.
And you are right that a 17-letter first name would create complications for you when living in the US, where very long names are not popular. Different government agencies and businesses will chop off the name at different places, making you unable to have all your IDs and accounts and registrations with the same first name across the board.