This answer assumes that your push has not yet succeeded, but you have made the commit. In this case, since no one else has seen your branch with the large commit, you should be safe in actually nuking that commit:
git reset --hard HEAD~1
This will remove the head commit, and after this you may continue your work. A slightly nicer, less risky option, would be to instead do a soft reset:
git reset --soft HEAD~1
This would leave the changes from the top commit in the stage. You could then unstage the large file, and recommit your work.
Note that if you have already made a successful push with that large commit, then you have a much bigger problem on your hands. Now, even if you revert that commit, the large file will still be a part of the history in the repo. In this case, you will need a tool like the BFG repo cleaner to remove the large file.