The board size preference now represents your preferred maximum size.
You can demonstrate this by setting "mini", for example, which will show a small board on desktop.
Then shrink your browser window - the board will shrink further to fill the available space.
Expand your browser back to its previous size, and the board will grow, but get no bigger than your preferred maximum, regardless of available space.
A future enhancement will be adding a slider control for more precise setting of this option.