These types of fire are not liked by most gas installers (and I was one before I retired and I also considered them unsafe!!), for good reason, and are certainly not suitable for DIY installation. For one they need a Flue Gas Analyser to check they are actually working correctly, not normally part of a DIYers tool kit!
With regards size and location of ventilation the only people who can answer this is the manufacturer of your chosen fire.
??? 'Cubic' is a measure of volume (ie contents of a box) while 'square' relates to the size of one side of the box.
You really meant 100 centimetres squared, but you need to measure size of room as this requirement is dependant on room size plus there is a minimum room size these fires can be fitted in plus there should be an opening window in the room the fire is fitted in.