Back your disagreement with data.
Whenever you disagree, base your reasoning on hard data and references. Disagreement based solely on a hunch is ineffective and weak. Instead, leverage metrics to support your stance, because numbers don’t lie.
If it is an engineering discussion, gather the numbers by building a quick prototype, or doing a quick load test. If you have already been through a similar situation, cite some existing research, explorations, or blogs to back your stance.
For a disagreement during a product or business call, gather numbers by setting up a quick A/B test or a quick customer survey. Again, you can always refer to things from your experience or spend time gathering concrete stats and resources around your thesis.
Backing your disagreement with data keeps the argument constructive and makes sure that the focus is on measurable outcomes and not subjective judgment. Always, quantify your concerns.