What are you trying to do with a 1098? Do you mean your Form 1040? Are you trying to mail your tax return? A standard business size envelope will be fine.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.
I suggest going to an office supply store with one of your blank forms and seeing what works.