QR code component using flex box

Just a question about best practice:

I've learned, that using absolute values like px for sizing content etc. is considered bad practice. But, I was tempted to use px instead of vh/vw for the card, especially because I'm using media-queries anyway. Is it still better to avoid absolute values or is it ok, that the element keeps the same size, when resizing the window? Or is it just a question of design?