World Size Edit

So I'm trying t create a server on Aternos, but when I'm creating a world, I can't change the Generator settings, I wanted t change the Max-world-Size to a lower size world, instead of the 29 million radius one, for a 100 thousand one, so I could lower the lag and to make it easier to explore, hope you can help me out

but I have the forge, I can't have pluggins