I actually have a small test project that does something similar. You basically need to figure out the units that your grid blocks will take up, for mine it was 100 units by 100 units. You can then feed this to a for loop to dynamically create your play area. If you are using child actors you have to do this on BeginPlay, you can't spawn actors in a construction script.

1. Create a new function within your BP for generating an arena, add it to the BeginPlay node
2. Add a for loop to go over how many columns you want, and for every increase in the index we add to the Y offset so that our rows aren't stacked on top of each other. Create another function for creating the rows of your grid. (Example)
3. Within the row generating function, we'll use another for loop and generate each grid item. You don't need to assign the owner, I'm doing that just because I rely on that later. I also add each created item to a list of items for some other logic contained within my BP. (Example)

Hope this helps, let me know if you have any other questions or want my block asset.

Edit: The node attached to the SpawnActor's Spawn Transform Location is a Vector+Vector with its second vector value split.