I don't know if it's possible to increase the grid size, but I do know that it's possible to increase the geometry scale, which lets you make a map that takes up double (or more) the space with the same grid size. Only problem is you'll have to shrink your current work by half (or more), which might cause some alignment issues for you.
Map > Map Properties > Geometry scale. Then increase it to make your map larger, if I remember right.