Thanks, that helps; I'd thought about variable numbers of columns based on the width of what they're contained in, but hadn't thought about the possibility of wanting to choose the number of columns to fit their content better. Mostly because I was imagining content like images that can be scaled or text that can be wrapped, rather than fixed-size content or content with a minimum necessary size.
Depending on the non-fixed column type (there are different rules for each type) you want to ensure their constraints are satified (typeically they are big enough to encompass the items).