Yes. You can customize the template of a board in these 2 ways.

1.By changing the board settings:
The settings icon is located on the top right of the board.

Using this option you can modify collage height, border, and space between images.

2. By hovering over the empty space between the images:
You will see a pink selection as shown below,

Clicking and dragging this pink selection will allow you to modify the size of each cell vertically and horizontally.