It would be easy for this Fantasy Fantasy realm to explode out of proportion and become a series of continents dominated by dozens of nations great and petty, infested with a dozen different races. Such an endeavor I feel is doomed to tail simply by the sheer size of the project. It becomes overlarge, and a 'white elephant' that would get along for a while before succumbing to the bane of all threads...apathy.
Therefore the boundaries of this new realm are going to be set down in the begining, and considered chiseled in stone. This may seem harsh to some readers but for considerations of practicality it is mandatory. The first consideration is that of the physical realm.
The easiest way to accomplish said task is to make the realm an island, bounded by ocean on all four sides. It is a viable idea, most early civilizations (Crete, Greece, etc) were founded on islands as they had natural defense against rampaging barbarians. Unfortunately this feels artificial, and severely limits what can be done simply to the size of an island, and as much as I like the word archipelago, games and game systems set in island chains inevitably make me shudder.
A single Continent on the other hand is too large, and invariably leads to the next step of the next continent, which leaves me searching for a middle ground between the two. And there is one, the subcontinent. There is only one on Earth, India.
This idea has taken a strong root in my mind. India gives alot of ideas to simply grow out of. It sets a basic mindframe of basic geography, as well as giving a real world example of a biosystem inhabited by a large number of animals not usually seen together, IE Elephants, Tigers, bears, wolves, crocodiles, cattle, sheep and goats, cobras and mongeese. Jungle Book without the singing animals and people in red diapers.
Therefore the starting slate is a subcontinent, well insulated from the outside world by barriers that are less inviting to explorers that wide expanses of ocean. The northern barrier would be mountains of the massive, sky-ripping variety created by the slow collision of the subcontinent with the main continent. The peaks remain snowclad year round and aside from ascetic monks, and yeti the region is unpopulated and harsh. Those who might survive cross these mountains very well would perish in an attempted return crossing.
The Eastern border is vague defined by deepening jungle inhabited by ferocious animals that have both large teeth, and poison (Snakes and tigers) as well as being loaded with a variety of lethal diseases carried by mosquitos and rodents. Expeditions into the east usually end with fits of delerium, or the belly of a predator.
The Western Border is an expanse of rocky desert that stretches on and on until there is nothing but blasted rock, scrub and the undying mountains, extensions of the Northern Wall. This would be a place of rapacious barbarians, stinging and poisonous monsters and evil spirits.
The south is the easiest. The coastline offers a panoramic view of a stormy and treacherous ocean, the Indian in our world. The idea of expansion beyond the natural borders is constrained by the fact that land is not so scarce as it is in our Eurocentric world view. Ships exist, but not much more than small fishing boats, and skiffs for pleasure boating by the nobility. India went through a considerable period where there was almost zero maritime activity. They are a land centered people, as opposed to the sea centered British, Spanish, or Italian.