How my design looks in Figma is completely different from how it looks on the browser.

I mean talking about font sizes, and the dimensions of elements such as cards, images, etc.

I want to know if there’s a solution to this? Like how do you determine the width and height of images, or cards confidently that it would look the exact same? Like if you have a two column hero section with the right column containing an image, how do you determine what the height of the image should be, and does it match completely in the site development stage?

Do you even fuss about the width and height of these elements. Cause I waste a lot of time perfecting it, and I want to know how the pros do it. 

Would mean a lot to me if you can guide me to the right direction. Thank you.