This is what we recommend based on your details:

Ceiling Height.png

2.4 - 2.6 m

Room:AreaSize.png

5 x 5

Based on a ceiling height between 2.4 - 2.6 m, and an area size of 5 x 5, we recommend a 18,000BTU unit. 

Some options:

Available in 18,000 BTU

Carrier XPower Gold

Toshiba High-Wall Inverter