I don't have a precise number, but as with most materials, PVC pipe is very strong in lateral compression. a 1.5" PVC Sched 40 pipe (standard plumbing pipe) can easily support over 200 lbs. I can stand on the end of a short (4") piece and not worry about it breaking. It's less strong when it is flexed, but that doesn't affect what you have planned.
More of your issue will be the plastic grate. I doubt you'll get any real specification on how much weight it will carry, but I would start with a pipe every 8-12" in a diamond pattern to support the grate. I'd imagine that would be fine to support the rock.
How high do you want it? The higher you go, the larger the pipe. Basically you're looking for stability (picture yourself standing on it. I would be comfortable standing on a 4" length of 1.5" pipe, but I certainly wouldn't try to balance on a 12" length.
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Is black iron pipe strong enough for spotter bars?
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