The USPS will offer two postmarks to commemorate the 2025 inauguration:
The dimensions are 2.72″ x 1.26″ 
This postmark measures 3.38″ x 1.0″
The Inauguration Day postmarks will be available only from Cancellation Services in Kansas City:
Inauguration Day Postmarks
Cancellation Services
8300 NE Underground Drive, Pillar 210
Kansas City MO 64144-0001
Send your envelopes and cards to this address, with sufficient postage on each to meet the first-class rate (such as a Forever stamp, or a combination equaling or exceeding 73¢)
How can I receive the cancel?
Send your cards and covers to the address shown.There is no charge for quantities of less than 50 but the “covers” must have sufficient postage to meet the first-class rate (73¢, or a Forever stamp).