Do you want a biome that literally stretches to the ends of the world? You can find out how here!
As discussed in my other post, the biome setting in Customized doesn't actually create worlds with just a single biome.
It is possible to create single biome worlds in superflat, but they are flat.
It is also possible to create single biome worlds in Customized, including the biome specific terrain, but it isn't exactly straight forward.
To do this, you need to find a world with the biome you want at the origin (0,0), then increase the biome size to a sufficiently high number like 50 (Biome size =30 will enlarge a block sized biome in a default world to the size of the whole world, so anything larger will be fine)
The hard part is finding the appropriate seeds.
In this post, I will find seeds for each biome to make them infinite.
All you need to do is copy the appropriate preset, and seed, and you will have a world with literally only one biome (or two if you want rivers)