I'm not 100% sure, but I think it extends 30,000,000 blocks out in each direction from 0,0, like in normal Minecraft worlds.
I don't know the exact size but there is definitely a limit active in the Frontier. Some time ago there was talk about staff building a new ring of outposts which would allow players to more easily travel further out. There have been comments shared that those outposts had been placed already, but in a location not reachable by players. Ergo: beyond a (world?) border. But border or not: the Frontier is huge
The frontier world is larger than neptune. Dozens of times larger than earth and hundreds of times larger than earths land area... soo, huge. https://www.pcgamesn.com/minecraft/just-how-big-minecraft-world-big-it-turns-out
The world border is located at +- 1 500 000 in each axis, meaning that the world border length is 3 million blocks. This means that the frontier has an area of 9x10^12 blocks
We have a limit currently set at +/- 1,500,000 in each direction. By game mechanics, it is expandable to 30,000,000. However, we have it lower to allow for new generation worlds to exist for future expansions.
