Ok, obviously I'm going to try to defend the size of Sirrilas, as I belive it's reasonable (or I wouldn't have made it that way). You say if it were a famous shrine like Ald Daedroth it would be reasonable, but what makes Ald Daedroth famous? It's not mentioned in lore, it's not mentioned in any other elder scrolls games, it's only famous because it's used in two quests in Morrowind. What evidence is there that says there are ANY famous mainland Daedric shrines? I've read most of the in game books, and though I know I haven't read them all, I haven't come across anything mentioning any mainland Daedric shrines, let alone anything mentioning one as well known or famous. Do you take this to mean there are no famous shrines on the mainland, or, much more likely, that Bethesda simply didn't take the time and effort to come up with lore for shrines it never planned to build? If we treat all of Bethesda's lore gaps as evidence of non-existance then we'd have to remove much of the things that make TR's landscape interesting and inspireing. We should stick to lore, of course, that's the whole point. But that doesn't mean we shouldn't be creative as well and fill the gaps Bethesda left.
Sirrilas has all the evidence on it's exterior of being an important shrine. First: the exterior of the shrine itself has three entrances, only two were actually built as part of the original shrine, the third is a cave door, but that implies to me at least that there used to be a third entrance there, which was destroyed and someone (the necromancer living in the ruinous passages maybe?) replaced it with another sort of door (Personally I'd prefer it if it were a 'dark hole' type door). Most mainland Daedric shrines only have one door, hardly any have three, this implies from the start that it's probubly a large shrine. Second: The shrine has a Velothi guard outpost on the road to it. NO other shrine in the game has a guard outpost on the road leading to it, this either implies to me that A: This shrine was very important to justify this, or B: the guard outpost was built later to keep whatever lives in the shrine from causing problems in the surrounding countryside. However I belive if the later were true it would probubly use Indoril architecture, which it doesn't. Third: There is a stronghold extremely close to Moriken, which even has it's own walkway up to a platform overlooking the shrine. This implies that the stronghold was built to guard the shrine, both due to it's close proximity, and the implication which can be drawn from the platform that it was built after or at the same time as Sirrilas. No other shrines in the game have a stronghold to guard them. Obviously that probubly wouldn't be the stronghold's only purpose, it certainly provided protection for the farms in the surrounding countryside as well, but it's location is certainly significant.
I drew the following conclusion based on the locations of these structures (and claimed them all to flesh it out). Sirrilas was once a very important shrine to Mephala, possibly one of the largest in Tamriel. The fact that it isn't mentioned in lore doesn't mean anything, as no mainland shrines are mentioned in lore. The stronghold was built to guard the shrine, as was the guard outpost, and was probubly a post at which noble-born troops were stationed allowing them, if they were so inclined, to avoid much actual combat while still having an honorable posting.
I belive Sirrilas should have at least two quests envolving it, possibly three, all of which I'd be more than happy to make myself, one would be a kill quest for the fighter's guild or mage's guild to kill the witch/necromancer living in the ruinous passages (very simple). The second would be a major quest for the Morag Tong envolving the shrine itself, which I haven't really come up with a plot for yet, but I will
The third would maybe be a shrine quest for Mephala. Anyway, as I said, this shrine is larger than most Vvardenfell shrines, but that doesn't mean it breaks lore, as there are no large mainland shrines mentioned in lore.
Anyway, read the readme for details!
, and lets say for example, who build the shrine could have been a cult that wanted to mark their achievements in the main land with this massive shrine, as well as a token of a major appreciation for mephala. 
