I used Gaea for a webgl project in Unity. The terrain can't be more than 2K in resolution and the best performance we found was at 512.
Don't know what you are using to build the world but you will want to look at the requirements for webgl in that specific program.
We had to use world streaming to get 1k to work well in webgl.
You can use objects and textures from other apps on the terrain but those come with their own limitations as well. For objects, we used impostors, which were essentially a 3D object baked to project onto 2D planes in the x, y, and z which allowed for greater performance and visuals from further away.
Textures were no larger than 2K and limited to 4 in Unity. Any larger and any more saw the project come to a crawl or even a halt.
I hope that helps.