These are the 5 sqm houses you can find in Tibia:
Theater Avenue 6c, Carlin 25k gold
Theater Avenue 6d, Carlin 25k gold
Outlaw Camp 10, Kazordoon 25k gold
Outlaw Camp 11, Kazordoon 50k gold
Outlaw Camp 4, Kazordoon 25k gold
Outlaw Camp 5, Kazordoon 50k gold
Outlaw Camp 6, Kazordoon 25k gold
Outlaw Camp 9, Kazordoon 50k gold
5 sqm is the tiniest house you can get in Tibia. Then there are plenty of 7 sqm houses. Easiest way to find what you're looking for is to simply go to tibia.com > Community > Houses (https://www.tibia.com/community/?subtopic=houses) and there you can sort your search by size. The benefit of doing this is that you can easily see if the house is empty or rented on your world.