Hi everyone, I'm a newbie and have no experience in this area. I'd like to ask why my photo was rejected, even though it's quite large. Could you give me some advice? Thank you

Large isn’t necessarily good. 

When you upscale bad pixels, it just creates more bad pixels.

This is what you’re competing with in Stock inventory. 

https://stock.adobe.com/search?k=daisy  3.6 million results.

Carefully read your Contributor User Guide for more tips.  Stock customers expect commercial quality artwork for use in professional projects.

• https://helpx.adobe.com/stock/contributor/help/photography-illustrations.html
• https://helpx.adobe.com/stock/contributor/help/editing-dos-and-dont.html

Hello,

I think focus could be an issue. i wonder about the DOF—I think it is too shallow.

I think the centre and petals should be more in focus in both images.

The size is not an issue. Both images need to be sharpened and the second image has a cropping error on the stem.