Increased stirrup diameter in some cases and yes reduced stirrup spacing in others. Your suggestion on increasing column dimension is however well noted ... but unfortunately sometimes Architectural constraints determine how much column dimensions can be increased (the ever continuing saga between engineers and architects
)
Also, Sometimes when one increases column dimensions, then huge plastic moments capacity develops in the column thus making it more difficult to satisfy capacity design requirement in shear.
In any case, I should expect the error message to disappear once the safety ratios for the governing case is greater than 1 ... correct?