Hello out there i was building my portfolio but i got stuck at the User10

which is : " The height of the welcome section should be equal to the height of the viewport ". Well i kind of figure out the exact heigh but to tell the truth just don’t know how. I kind of keep put it number to the height of my welcome session until i get the right one.

So my question is this How to actually know the exact height of the viewport? then next time it can be easy for me to adjust that. Ok thanks

Note: Sorry for my english is just my 5th languages