The brush size you need should be determined by the size of the specific area you want to paint. If that area is a foot long and 6″ wide, you would use a bigger brush.

If the area right next to the previous, bigger one is 1/4″ long and 1/16″ wide, you use something like a size 0 round, i.e. a smaller brush.

You get as much detail as you’d like, as long as you put in the time and effort!

It should be pretty simple to take your source photo or painting, draw it out on your new canvas, and study the larger/smaller areas and determine brush size then and there.

I can’t (nor could anyone else really) give you specific brush recommendations, as we don’t know the source material you wish to paint – big areas of color? Tons of tiny detail? etc. I know liquitex has some really large synthetics, and I prefer Escoda Prado synthetics, but I don’t know how big they go (but they ain’t cheap, even at ‘normal’ sizes).

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…