I just finished one today where the subject is located on a 40 acre parcel but the lender says to use only 10 acres.
They did not tell me what to use (no legal description) so I put the assumption in the report that it was a typical 10 acre; 330 by 1320. These are common in my market.
What you have will be a Hypothetical Appraisal based on an assumption that is not true. You will want to make sure that you state that it is a Hypothetical Appraisal and caution the user that the value contained in the report is not reflective of the actual value nor have any relationship to the way the property is actually constituted.
Here's a statement that I put in the Addenda:
COMMENTS ON HYPOTHETICAL SITE USED IN THIS APPRAISAL:
AT THE SPECIFIC REQUEST OF THE LENDER A HYPOTHETICAL SITE OF 10 ACRES IS CONSIDERED IN THE REPORT. THE ASSUMED PARCEL IS 330' BY 1320' WITH 330' OF ROAD FRONTAGE. NO LEGAL DESCRIPTION WAS PROVIDED. THE USER OF THIS REPORT IS CAUTIONED THAT THE VALUE CONTAINED HEREIN, BECAUSE OF THE HYPOTHETICAL SITE, MAY NOT REFLECT SUBJECT'S ACTUAL VALUE.
You might want to put it in the FIRREA/USPAP page also.
The important thing is to make sure you state in the report, and perhaps several times, is that what you are appraising is not reality and that the value conclusion is not reflective of subject's value.