I checked the website, for uploading... and I can't place my upcoming game in the right place, because it's gonna have 23 rooms. Is it medium or long then?
I always took full length to mean the equivalent of a professional game. I would say length should be measured in hours of gameplay rather than in number of rooms. As it says on the games page, medium games are up to one hour's gameplay.
so it's a medium game! ;)
You don't need to consider placing it in the full length categories unless it has more than 35-40 rooms.
Although, as GG stated, there are other more important factors, such as gameplay length and overall quality.