I know her in-game size isn't accurate to her size in lore, but The Terror Beneath video isn't a whole lot to go off of. The LOL Wiki describes her being the size of a house more or less, but it isn't cited and a rundown on her release Q&A reveals no Rioter actually gave a size estimate; maybe I've just miss that one post that says it. But can a Wrioter give us a ballpark range? 15-18 meters, going off The Terror Beneath and her model?
Small T-Rex. Mammoth. Whale Shark. Large not huge.
They did give a size comparison on the burrow holes on her promo pages:
http://na.leagueoflegends.com/en/creative-spotlight/reksai-scourge-desert
The average-sized specimen: http://vignette4.wikia.nocookie.net/leagueoflegends/images/7/7e/Rek'Sai_-_Scourge_of_the_Desert_04.png/revision/latest?cb=20141211015801
A Xer'Sai the size of your pet hound: http://vignette2.wikia.nocookie.net/leagueoflegends/images/9/93/Rek'Sai_-_Scourge_of_the_Desert_07.png/revision/latest?cb=20141211015803
I would've said she was grey whale sized, because that looks accurate to me:
http://www.statesymbolsusa.org/sites/statesymbolsusa.org/files/gray-whale-size.jpg
They grow to be around fifteen meters long (50ish feet) and weigh in around 40ish tons.
Tough to say. But going off the Terror Beneath.
Lets say the camel like creatures are the same size as a dromedary camel. Which on avg is ~ 1,100 lbs, ~10ft long and ~7ft for a adult male. When she attacks the creature she appears at least 2.5-3x as tall and 5-6x as massive.
So using the dromedary size as the base. I would say you're looking at a animal thats about 18ft - 21ft tall (at the dorsal). Probably about 50-60ft long to her tail. Since she appears to be an invetebrate, her weight would be much harder to determine.
So basically, assuming shes around 18+ft tall and 50+ft long, she would be very similar in size to Spinosaurus who is estimated to be very similar dimensions.
https://upload.wikimedia.org/wikipedia/commons/a/ae/Largesttheropods.png
