Jedi Master of Orion wrote...
Imperium used to rule the Anderfels, Seheron (but not Par Vollen), Rivain, Antiva, Nevarra, Orlais, the Free Marches and much of Ferelden. The combined size of their holdings is maybe around 6 times the size of the Imperium today so they've lost maybe 80-85% of their territory.
Ferelden and what would later become Orlais and part of the Free Marches were liberated by Andraste's Exalted March. After Andraste's death, Hesserion and Maferath essentially divided rule of of the two halves of Thedas between themselves. But the barbarians seemed to be winning before Andraste was betrayed. The Valarian Fields were right outside Minrathous.
The rest of the Free Marches, Rivain and the then city state of Antiva rebelled against the Imperium in the century and a half that followed Hesserion's death. And the Imperium abandoned the Anderfels to the darkspawn as soon as the Second Blight began.
Thedas once meant all lands outside the Tevinter Imperium but it was eventually came to mean the continent after most of it became "Thedas". Because of that I doubt the Imperium ever controlled any lands beyond the continent, especially since they never controlled places like Par Vollen or the Kocari Wilds.
They built a fortress on the edge of the wilds though, to ward off Barbarian Incursion.
Perhaps if the first blight hadn't be unleashed they would have been able to conqueror the entirity of it.
And Par Vollen is much the same considering it was surrounded all sides by a thedas controlled by the Imperium further more i find it implausible considering the length of the former deep roads networks that all led to cities within mainland Thedas.
And considering my prior point with them establishing contact and trading agreements with the imperium at the same time, its likely the imperium had outposts above these cities to ferry goods and supplies back to the imperium, i just find it odd that the assumption would be they were all within the entirity of mapped thedas.