One of the elements of excellence in design is undoubtedly the dimensions. Even in photos with random brush strokes, there is a certain measure and math.
Not everyone understands this math. But it is often obvious. Designs can be made with thousands of different combinations in thousands of different grid systems.
A good course on Grid has helped me to open my horizons in this regard.
I recommend you to review the grid courses of George Bokhua
Another factor that is as important as the dimensions are colors. If you need a color palette when designing a postcard, you can follow this page.
I prepared this page last year. They have color palette inspiration. I got all these inspiration from the covers of famous albums. I wanted the colors taken from the covers of the songs that inspire millions of people to inspire new people again.
The idea of the recycling of inspiration made me very excited.
If you want to design a postcard, you may want to examine what other artists have done. Here is a postcard design that you will get some inspiration for.
Although social channels provide instant and 24-hour service for us to reach each other, we still have not stopped throwing postcards.
Postcards are still sold in tourist places. We send the postcard to our loved ones from the places visited.
This is still done by people who have experienced living in the old world. I do not know who will continue this tradition in the next 10 years.
I am 27 years old and when I was young I once posted a postcard.
Will those who are 27 years old 10 years later be aware of this tradition? I do not think so.
I mentioned above of standard postcard size.
Good luck with your design.