Setting dimentions

Good morning everyone!

I have a question about the box model topic. While I’m getting to grips quite comfortably with everything else, the bit that gets me is choosing dimensions. a lot of the tasks ask you to input dimensions but don’t give any indication as to why one dimension works better than another because when I come to build my website I find I struggle as to what dimensions to put.

If anyone could point me to any resources or offer any advice that would be greatly appreciated!

Many thanks,

Alex