This is a doctrinally difficult case, since it combines two distinct lines of precedent. Perhaps for this reason, its holding is at least a bit off.
Two general principles come into play. First, the determination of whether a particular governmental act constitutes a "taking" that constitutionally requires just compensation is made by a judge. A judge -- not a jury -- decides whether an uncompensated taking is permissible, and in an appropriate case looks at (among other things) the rough proportionality between a particular development and the governmental requirements for that development. For example, when a developer wants to develop X property, but the government requires the developer to dedicate a certain percentage of that property for roads, school and the like that will be required by the development, it's a judge who decides whether the conditions are proportional.
By contrast, in an eminent domain case, a jury -- not a judge -- decides the value of the property. That's true even if certain governmental acts that might affect the value of the property are at issue. For example, if a property is currently zoned agricultural, and the government condemns it, the landowner has a right to try to prove that the property is worth more than agricultural land because it could have easily gotten the property rezoned for residential development had it attempted to do so. The possibility of rezoning is part of the value of the land. So a jury decides the issue.
The complexity in this case arises from the confluence of these two principles. Who decides the issue when a landowner says that the property is worth $X because even if the government attempted to condition the development of the property on dedication of a certain portion (for roads and the like), that attempt would be unconstitutional? Valuation is usually decided by a jury. By contrast, constitutionality is usually decided by a judge. Which principle governs?
The Court of Appeal holds that because the constitutional issue involves determinations of facts, a jury gets to decide the issue. In Justice King's words, "a jury must be allowed to determine whether, and to what extent, the  take . . . is roughly proportionate to the Stamper Property's anticipated impacts on area traffic if and when the Stamper Property is developed." A jury, then, gets to determine "whether all or any part of the 1.66-acre take could be constitutionally imposed as a dedication condition on development."
At best, I think that statement's too broad. In a way that's critically important.
It's possible that Justice King is correct that a jury gets to make underlying predicate findings of fact. True, in a pure constitutional case, a judge (rather than a jury) gets to make those predicate factual findings as part of the constitutional analysis. But perhaps the Court of Appeal is right that valuation of eminent domain cases is assigned (at least in California) to a jury. So predicate factual questions get transferred to a jury.
But even if that's the case, the Court of Appeal errs when it holds that the dispositive legal issue -- whether an uncompensated taking would violate the Constitution -- gets decided by the jury. On that issue, a judge, not a jury, is the relevant factfinder.
Yes, a jury might perhaps properly be called upon to make certain findings of fact relevant to that legal determination. For example, a jury might be asked "How much traffic will development of the Stamper Property cause?" and/or "What percentage of the traffic on the road that runs through the Stamper Property will consist of traffic generated by the Property itself, as opposed to traffic from some other source?" Those are factual questions, and indeed they're relevant to whether there's rough proportionality between the taking and the approval of the development.
But whether those predicate facts satisfy the constitutional requirement of "rough proportionality" is purely a question of law. The jury decides the facts. It doesn't decide the law. Judges do that. Whether a certain set of facts suffices to satisfy the dictates of the Constitution is purely a legal issue, subject to initial determination by a trial court and, thereafter, de novo review on appeal. Juries don't get deference on that issue. Nor do they get to decide the issue in the first instance. That's for the judge.
So, yes, a jury could properly be instructed to find certain predicate facts. Would the City of Perris try to take a portion of the property? How much traffic would be generated? Stuff like that. But a jury is by no means "allowed to determine whether, and to what extent, the  take is roughly proportionate" or whether the take "could be constitutionally imposed as a dedication condition on development." That ultimate fact is for the judge. Informed, perhaps, by the jury's special findings of fact.
That's a meaningful difference. Because courts determine the constitutionality of a governmental act. Not juries.