To make it smaller than default you move your cursor to the upper-right of the board, click-hold the resize icon (double-sided arrow) and drag left for a smaller board and when it isn't at maximum size, right to make it bigger.
The closest thing to fullscreen is Focus mode. That is the four-sided arrow under the resize icon when you move your cursor over that section.
How do I make the board Fullscreen size?
How do I make my board different sizes?