What is "Photoshop Default Size" ?

My PhotoShop CC 2017 defaults to 300ppi, so I have 1890x1417 but monitors were 72 ppi, so thats where 454x340 comes from. I'm just looking at the shape/ratio of the canvas, not the exact pixels. Given the other answers, and knowing that the ratio is 4:3 (1.33333...)  I would expect that it was the format for digital (not for print) photos and art. 4:3 was everywhere until the mid 2000-2010 decade. Think TV's, video game title screens, CRT monitors (using the images as backgrounds, would be a bit pixelated since it's smaller than the screen), and would fit in the internet browser window nicely (if the aspect ratio of the window wasn't changed). It's basically the same as using 16:9 for digital art now.

It might depend on the localized version of photoshop, but on the us english versions (since photoshop 7) the default photoshop size is 504 pixels x 360 pixels or 7 x 5 inches at a resolution of 72.