How do you size you map...maybe I am thinking wrong

Okay, working on a Map with a Grid Overlay.     My thought is to size the map such that at 100% on my screen (laptop) I can see the width of the map and need to scroll moderatley in the vertical direction to see the entire map.     I then go and use Align to Grid and it ends up making the map larger.     It seems to me that I want to set the pixel hight of the individual squares, and perhaps iterate to find the best height.       Maybe I am thinking wrong, but to me , one should size the map and then align to grid is the order.     Any help is appreciated. 

By default, each square on Roll20's grid (when at 100% zoom) is 70x70 pixels.  I am not aware if this can be changed, but generally when making maps I try to keep this measurement in mind.

Thanks Riley!!    At least I understand how it works, so now I can format my maps accordingly.  

When I import maps into roll20, the dimensions of the map are always slightly off. Keep that in mind, you can fix this under the advanced tab. I always resize maps so they have either a 25x25 px grid, or 50x50, crop your map at a grid line and import to roll20 Set the dimensions to the image size and then set your grid to 25x25 or 50x50 and you are done.