RenderTexture are 3D camera rendered bitmap pictures loaded in the graphic card memory so they can be mapped on a geometry surface. Usually, texture are mapped through a Material. Keep in mind that WebGL, as an implementation of OpenGL, shares the same kind of limitations : textures sources should be power of 2 (even if here, Three.js is doing the job of resizing for you if they're not).
the parameters object
the width of this texture
the height of this texture
the texture format to use (see https://threejs.org/docs/#api/constants/Textures > Formats)