By adjusting their reflection coefficients, intelligent reflecting surfaces (IRSs) can reshape the channels between a base station (BS) and user equipment (UE). Existing studies mainly optimize IRS phase-shifts for communication objectives, leaving the physical attainability of prescribed IRS-reflected channels largely unexplored. This paper investigates channel customization with a phase-only IRS under three progressively relaxed criteria. Observing that the customization mean-squared error (MSE) has the same mathematical form as the signal MSE of a point-to-point multiple-input multiple-output system, we construct a counterpart problem to characterize individual singular-coordinate gain limits and the condition for attaining the amplitude-matching upper bound. To address multi-stream phase conflicts, we analyze principal contribution area (PCA) overlap and derive sufficient array-size conditions for disjoint PCAs under a specified focused-vector construction with parallel uniform linear arrays. We also derive necessary conditions from the two propagation links' singular values and an array-size restriction under the parallel-array Fresnel model. Both the restriction and the construction involve the BS--IRS array-size product. Numerical results show that jointly selecting right singular vectors and IRS phase-shifts reduces mean customization MSE, whereas gains exceeding propagation spectral limits cannot be realized exactly. These results provide design guidance and highlight spectral limits of phase-only IRS channel customization.

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