The question are several:
How heavy ?
That important for Movement and usability, unit of 150 metric Tons have issue cross bridges or sink into soft earth.
How Fast ?
Mass is also issue on fast Movement if that Bot is heavy armoured it can limit fast movement of arms and legs
it you Bot move only 5 km/h vs Tank with 50km/h the tank will win, also a A-10 with BRUMMMMMM
How to power that dam thing ?
Most Sci-if example have Nuclear Reactors, fusion engine, Hell even genetic engineer organic body of Giant human
or just use Magic to operate. (authors don't care)
If we install nuclear reactor, it again a Mass problem this time combine with Chernobyl...
let use Gasturbine that burn multi fuels, give best Power vs Mass Ratio, mean you have power to operate a Light Bot
only issues: booze fuel like hell, overheat problems and a very big IR signal for Enemies
until they the Invent the Minovsky particles or other Clark Tech to make it work
Combat Robots will be remove operating small Drones...