This is indeed the question that one has to answer for oneself. If a picture shall be published on the web only it doesn't matter. Yet as soon as one has a print in mind this question is important from the beginning on.

The print companies want to know the size in addition.

Not every picture looks great super blown up. The size of a postcard might be a bit modest.

When starting a picture series this question must be answered before taking the pictures. It's easy to create a canvas in Photoshop. This is then the frame. Within a series not every picture should have another size. They must fit together.

If one doesn't answer the question in the beginning the issues come in the end. Either one has to cut the picture later or there will be a white frame around it.

Sizes are also a matter of taste. Some like the square format. It became very popular. Some picture require a portrait format others a landscape format. A classic format is also the Hollywood format.

The size of a picture is not something that shall happen accidentally.

The above picture is 20 x 25 cm.