Ice changes a drink as it chills and melts. Its size and shape also change how the drink sits in the glass. A single large cube or sphere leaves a different impression from several smaller cubes.
Fit comes first
Measure the opening of your glass before buying a mold. A piece that looks right on the tray still has to pass through the rim and leave room for the drink. Allow space for the liquid level to rise when the ice goes in.
Experiment with the pour
Some drinks welcome dilution; others are best adjusted a little at a time. Try the same drink with different ice and notice what you prefer. Clean, fresh ice and a mold that fits your freezer are practical places to start.