What size(s) flats should I make? Upcycling flannel

I’m a first time pregnant mom due in December, I’m from a cloth diapering family (yay!) and I’ve been blessed with a ton of used diapers for free or super cheap off fb marketplace… in fact probably more than I need.

Despite having tons of styles (including prefolds, pockets, fitted, all-in-two, covers of various sizes, inserts of various fabrics and sizes) I also really want to try out flats.

Right now for tiny stuff I have used cloth-eez nb prefolds, probably 15-20 of them, plus 4 cloth-eez newborn workhorses, and 8 older (1990s) flannel small fitteds. The fitteds are quite small but a bit larger than the nb workhorses. I have both old fashioned style diaper pins and snappis.

I have a plethora of 100% cotton flannel receiving blankets in various levels of used condition (from like new to pretty worn) and have been thinking of using some of them to make cloth wipes and flats with. My grandmother who used flats with her own babies has offered to help me since I’m not a very experienced sewist.
I’m just wondering about size!
What dimensions should I make flannel flats to be?
Should I make more than one size?
Is there a size I can make them all that will last from newborn-hood to potty training?