I had to scale up a design once from an original on two sheets of illustration board
which won me the contract.
The original design was then scaled up to a final size of 100 feet x 12 feet high..
It's hard to see how big that is until you see a figure for comparison:
generally I used 4 inch plastic pots with lids, and two inch flat gesso brushes, for the finer details one inch flats... or brushes with the ends cut off at an angle... some people call them 'angular flats'
These gave me a good range of marks I could make at that size.. the background colour was painted in with a roller.
After that big mural, anything like 60 inches wide seems quite small..
On a 60 inch wide painting I would still mostly use only up to maybe three quarter inch filberts or flats, bristle or taklon,
on a painting that size.... depending on what marks and effects are needed...
maybe I would also sometimes use a one or two-inch hogshair flat brush
though mostly, I suspect I would use brushes smaller than three quarter inch...