What should be the standard size of QR codes on business cards so that they can get scan easily?
I would question if a QR code on a business card is necessary. Business cards aren't generally "advertisements". The purpose of a business card is to provide contact information. If the QR code merely repeats that same information, I'm not so sure it's beneficial. Imagine someone taking the time to grab the phone, scan the code, and see that it merely repeats the same data they are staring at on the card. No added value.
If the card is an advertisement - with the sole purpose of getting the reader to scan the code - make the code as large as possible while still being aesthetically pleasing.
This depends on the specific QR code. As a QR code is a visual representation of a string of text; the size of the QR code depends on the length of the URL/text. If you got a very long URL you may want to use a URL shortening tool.
If you do this in Indesign (which is the appropriate tool for biz card creation, whereas photoshop is for photo editing and figma is for UI design) it will generate the QR code at the recommended size (I think once got told rule of thumb not smaller than 70% of that). Meaning recommended 0.9mm or at least 0.6mm width of pixel. But of corse also depends on distance of scanning so on poster it should be bigger than on biz card.
So long story short, there is no way to tell for sure apart from printing and testing it. And for the most part, if you think how good it still can be printed and if the squares are visible when viewed by a camera, you should be able to guess quite good if it should work.