How Big Is the Kingdom?
Roughly around half the size of the US, it's not completely theirs, either. There are wild and badlands at the edges of the Kingdom. But it's not inhabited by any other large nation or kingdom.
Are There Other Kingdoms?
Yes! However, the Sacred Seed Kingdom is very isolationist. They're aware that there are other people's out there-- and even more than the roaming nomads at their borders who terrify them... But they mostly don't have much contact with them. There are rumors of foreign countries with different types of magic, but most non-elemental magic is looked upon with suspicion or downright hostility.
What's the Tech Level?
Frontier / Wild West kind of... The very very rich Seedless have access to prototypes of cars, and other mechanical wonders. There's telegraphs and maybe even telephones. However, technology containing lead and copper makes the Seeded sick to be around it. So technology has been hampered in some ways. The Seeded control the vast majority of the wealth, and they don't fund things that will make them sick.
Do Outside Nations of Tech?
Maybe. That might be part of why the Sacred Seeds are so isolationist. Legend has it that they once sent a delegation across the sea by boat to explore the world... For only one survive to come back, terribly ravaged by an unknown sickness, and tell tales of smog choked skies, and ash covered grounds...
Do Outside Nations Have Seeded or Seedless?
No, that is only found in the Kingdom of the Sacred Seed. Whatever magic the outside world has comes from some other source...