Because taste is completely personal, I feel that my job is done as an interior designer when I have nailed scale and proportion. By that I mean, all the furniture fits in the space, nothing feels crowded, and it feels comfortable to be in and easy to move around the space. The room should not be so full of stuff that people cannot breathe.
A king sized bed will not fit in all bedrooms. Some bedrooms are just too small for the sleepers to get in and out of bed. Another item that won’t always fit is a sectional.
How do designers determine what fits? We measure and we do floor plans. We apply standards that guide how much space is necessary between objects and for flow through space. Good designers have an innate understanding of scale and proportion based on training and experience.