Hopefully the description for the file before you download will mention
the providers it is rigged for.
Once you download, you can search the file with a hex editor for the
nipper string or A3E, which indicates the embedded rom10 is present.
If present, the firmware may or may not be rigged for your provider.
You would have to load it into the receiver to see if it works.
Rigged firmware size will be the same as legit firmware.
Avoid the manufacturers websites, unless you have to revert back to
Always backup your firmware first, before you start to experiment.
Most of the time this means having a good jtag adapter and program.
I use jkeys with a .def file tailored to the receiver I am using.