Show a patch indicating the issue? Syphon sends the native dimensions of the input texture. If you are manually scaling the geometry, i.e., sending a 512×512 texture to a video plane @scale 2. 1 1 (so it looks as if its 1024×512), Syphon will send the 512×512 since its the textures actual dimensions.
If you see any behavior indicating otherwise, please report a bug with a reproducible test case.