How to measure and hang a grid gallery wall! Dining room art with affordable custom frames & hardware

I'm making some big changes to our dining room and those include filling the big, empty wall with some very special art! I'm a big believe in BIG art (or at least appropriately scaled for the space), but a mix of types throughout a home. For example, maybe a huge framed canvas on one wall, but a set of smaller, grouped black and white photos on another.

In our case, I want a big piece above our fireplace, so I wanted to do a gallery wall in the dining room. But, I love order and simpli...