This may not be implemented yet but we've been asked to report everything we can to help with the new B2.8 RPR plug-in.
There seems to be no way to scale textures with RPR as of version 2.0.10.
I have created a video to demonstrate the node tree setup in EEVEE, Cycles and in RPR. With RPR the texture is always rendered at the same scale. With the other two render engines the Texture Coordinate node allows the scale of the texture to be adjusted accordingly.
The node which used to perform this function in RPR for Blender 2.79 was called RPR Texture Mapping.
I may also be doing something wrong because I've worked with RPR for so long in the old Blender I've not kept up with the nodes from Cycles / EEVEE! So I'm catching up
Link to the video: Texture Coordinates Problem.