Last name doesn't fit on space provided

Enter as much of your name as the space allows.  Just be sure that your Social Security number matches that name.  The IRS matches the SSN with only the first few letters of the name.

Thanks @TomK2023, the only part about your answer that I find slightly confusing is "Just be sure that your Social Security number matches that name." as per the question, we are not able to write down our entire last name as it appears on our SSN. Could you clarify what you meant by that sentence please?

The IRS computer only checks the first three letters of the last name so that is what must match your social security card.

Thanks so much!