Does the physical size of the stamp matter?

No, the stamp's physical dimensions do not change how a mailpiece is processed or priced. What matters is the postage value and correct placement, not whether the stamp is large, small, or commemorative.

• Value: the postage must cover the mailpiece's size, weight, and class.
• Placement: put it in the upper right corner of the address side.
• Format: a standard stamp, an oversized commemorative, or metered postage all work the same way.

Focus on paying the right rate and leaving the upper-right corner clear, and the stamp's look is entirely up to you.