I don't have a geography degree, so take this with a grain of salt. On the flip side, I'm pretty good at math, so I'm going to try and extrapolate Runeterra's size and locations(not the climate, landforms and stuff like that, just size and location). I will try to use canon lore for data as much as I can.
All image comparisons are used using my pointer fingers(which are exactly 1.5 cm wide).
I read somewhere in a Ask Riot that Runeterra is very Earthlike in shape("oblate spheroid"), so I'm going to be using Earth's dimensions for numerical basic values.
On the Wiki it says that the continents are in the Northern Hemisphere, so everything will be north of 0 N.
It also says that the current Runeterra is 1/6 of the entirety, so it would have an area of 510.1 million/6, which is 85 million sq km.
Land area, presuming a Earthlike amount of 30%, the amount of total land is 25.5 million sq km.
Runeterra's got about 50% more land than Earth(due to the seas not being super big), so the land would instead be 38.25 million sq km.
Since Piltover's climate is described as balmy, we can put it at 30N, since its climate resembles what is found on earth at that latitude(the Mediterranean).
Piltovans also look pretty white, so they can't be too far south. It may also be part of their culture to stay inside more. This explains why Ionians are a little bit darker.
Due to the Freljord, the north tip of Demacia, and northern Ionia being snowy, it's going to be further than at least 50N(the Encyclopedia Britannica says snow occurs at latitudes larger than 35).
Further north than 64 N on Earth, we don't find any large cities, so the northern tip of the Freljord would be 66.5N. The last major settlement would be the Frostguard Citadel.
With these approximated calculations, we can find that the south tip of Icathia to be near if not on the Equator.
Since where you mark as 0 doesn't matter for longitude, I'll just say Piltover lies on 0.
Since Valoran is about 1.5x times as wide as it is tall, the west edge of Valoran would be 36.5 x 1.5= 54. 75 . Accounting for the little bit of Valoran east of Piltover, it can be estimated that the west edge of Valoran is 50 W, the eastern edge is 5 E.
Using this calculator(https://stevemorse.org/jcal/latlon.php/nearest/distance.php), I roughly got Valoran's size to be 4200 x 4000 km(the shape of the continent is uneven and the numbers aren't nice, so it's an approximated value). That gives us 16 million and 800 thousand. Previously, I had calculated the land in 1/6th of Runeterra to be 38.25 million sq km. This gives 21.45 remaining for Ionia and Shurima
The southern tip of Icathia being on the Equator and Piltover being around 30N gives us a nice number to calculate Shurima's size. The width, compared to Valoran, puts Targon at 30 W. Kumungu(which ends at around the same longitude as Valoran's east coast), is at 5 E. Shurima's much more like a rectangle, so the result is going to be much more accurate. Shurima's size is 3830 x 3300(adjusted, due to Icathia sticking out) km. Or 12.64 million km squared.
21.45- 12.64 = 8.81.
This means that Ionia is around 7.5-8 million square kilometers, since we have to take some away for Bilgewater and the Shadow Isles, as well as any outlying islands.
Provided that Bilgewater and the Shadow Isles look to be the same size, they would be about 0.4-0.6 million sq km each.
Phew, that was a lot of work. Lemme know what you guys think!!