How do you change from 10 meter boards to 5 meter?

Did a search, but couldn't find anything which I thought was weird: surely someone must have asked this in the past. I don't even know what size the default is.

Next to the button of add ground. To the right, its 10m default, click it to make it 5m.

In the F1 Topology tab

Here's a bit of warning from experience with this.

It's a great idea when you're first starting before placing textures. Once you've placed textures, such as when updating an older route, you can end up with broken texture mappings should you decide to update the older 10 meter grid to 5 meter. You may also want to consider this option for up close because the 5 meter grid is 4 times higher resolution, not twice, regarding the polygons needed to generate the ground mesh. Don't get me wrong here; it's great for areas where you need the finer mesh, but consider using 10 meter for the distance where seeing an accurate cut isn't necessary.

Thank you both!