Reaper has some smaller 28mm robots that look great in 15mm.
One of the packs is the Tool Bots.http://www.reapermini.com/Miniatures/Sci-Fi/latest/50015
I converted them to Care Bears.
The look great next to my Rebel Mini's Earth force guys.
The pack had some weapons that I didn't want to use. One was a saw and the other had a gun. I put one together with the backpack and the claw but he didn't feel medical enough.
I used a 6mm turret I had in the bits box as an arm. It had a big sensor on it and two guns. It looked like a medical scanner/manipulator to me. The backpacks are GW radio backpacks with the antenna removed.
I think the came out looking great.
I also have some unfinished bots from another reaper find.http://www.reapermini.com/Miniatures/Sci-Fi/latest/50164
I haven't done anything with them yet other then base and prime. They fit well with the 15mm figs. The small ones are about 8 or 9 scale feet and the big one is closer to 12.
Still deciding how to use them in my army. I don't have very many SAWs or RPGs so I'm thinking about replacing those support units with an attached robot.