Hello. You might have not adhered to the specified dimensions for signature which is why it is small and vertical. Though you have not mentioned which exam you are talking about but since you said that you have submitted already so changes might not be done now. But usually all the exams have a correction window date of which people can correct their mistakes (photo, signature, information etc) so you can wait for that date to be declared and you can then upload your new signature with the specified dimensions (check the official website of the exam and see the mentioned dimension).

Hope this helps. For further queries feel free to comment down below.

Good luck!