First off, Data outright says that it's a planet during his briefing to Picard:
On top of that, the official novelisation for the film "Nemesis" makes it abundantly clear that Remus is indeed a planet, rather than any sort of planetoid or moon.
Romulus and Remus-twins, but not equals, for while Romulus enjoyed a
regular night and day, Remus was placed too close to the sun. Half of
the planet baked constantly, a sere, unlivable desert; half remained
in constant night, and on that half, the population dwelled, and over
time had evolved into hideous, light-blinded creatures. The Remans had
already proven themselves inferior by living for centuries as slaves
under Romulan rule. Had they been worthy of the rights and status
accorded Romulans, they would have fought for them.
The script also offers this information
EXT. REMAN HOMEWORLD - NIGHT
On a stark, desert planet with monolithic mountains and harsh crags
shooting upward. The only light in this dark netherworld comes from
the flames that accompany the hellish mining operations everywhere
INT. SENATE CHAMBER - NIGHT
We observe the Romulan crest, an imposing bird-of-prey clutching a
planet in either claw, not unlike the KAOS emblem from "Get Smart,"
that dominates a wall. As we hear:
SHINZON (V.O.): Consider it The great symbol of the Empire... But the bird-of-prey holds planets. Romulus, Remus. Their destinies
And then we see him... SHINZON!
SHINZON: Yet for generations one of those planets has been without a voice. We will be silent no longer.
There's also an interview here with Ron Perlman who portrayed Viceroy Vkruk:
Perlman: The Viceroy and he met in sort of a mining situation on the
planet Remus. The Viceroy is Reman, and [Shinzon] was a young boy at
the time. We were exiled to this mine on this dark planet. I don't do
well in light, but I took some sort of control over his development,
[his] evolution into this man who eventually came to power over
Romulus and Remus... He's very mysterious - he's like an iceberg, you
can only see one-eighth of him and I like that, playing what's not
seen and what's not explained. The script [by John Logan] is really
good, written by a world-class screenwriter. It's not predictable,
it's not obvious, it's very well-realised. That, to me, is key in