The docs say explicitly that floor dimensions are exported, so I think you'll need to open a TAC case to see if there's a known bug for this.
step 5:
https://www.cisco.com/c/en/us/td/docs/cloud-systems-management/network-automation-and-management/dna-center/1-3/user_guide/b_cisco_dna_center_ug_1_3/b_cisco_dna_center_ug_1_3_chapter_0110.html"Next, export maps from Cisco Prime Infrastructure. This downloads map information such as floor dimension and calibration information like RF attenuation model that has been applied to each floor in Cisco Prime Infrastructure. "
If you're feeling brave, you might be able to peak into the file itself to see if the dimension data is there. I think it's contained in a CSV file.