Strength does not matter
However, metal (and other high tech materials) is significantly stronger than organic tissue.
Well that does not matter. Once we reach the max size about what is tenable for a machine, the square-cube law is just as valid. And even if metals are stronger than some/most organic materials (not all of them, nota bene) they are also more dense and — as such — heavier. When it comes to strength per unit of weight, metals rarely offers a significant advantage over organic materials. And when we throw price into the equation as well, then we quickly find that materials that has a good strength-to-weight ratio rarely comes cheap.
Also the real showstopper is not strength, but bulky energy storage and locomotion. Why are humanoid mechs — even in just the Puny Human size-category — coming into existence only now and not before? Because we have had no way of creating locomotive elements and energy storage small enough to fit these things. Such elements were all too heavy and too large, and they still are. A human can keep going for a lot longer than a humanoid mech, and our muscles are a lot leaner and compact than ditto for mechs.
We are always are at the limit of how big we can make machines, even out of metal. And looking at where we are today, we still have a long way to go before we make mechs that are as fast as cheetas; that are as tall as giraffes; that fly as good as birds(*); that have the strength, versitility and size of elephants. So organic materials are — still — well in the lead over human metal mechs.
...technology marches on. And it does so at a pace that always leaves the human mind coming running after it, desperately trying to keep up, with breath in throat and flabbergasted at what technology has achieved.
What is the upper size limit for a mech? No-one knows. No-one can know. I am sorry but your question has no answer because it would require the forseeing of technology and concepts that do not exist yet. If we could say "Here is where the upper limit for future mechs will be", then we would be magic fortune tellers.
(*) Actually we are becoming quite good at flying. Drones have made some amazing progress the last few years. But there we use plastics and composites, not metals chiefly