I'm not too sure on size because I'm not at home at the moment, it will probably be attached to brick work and might only be limited to a small section of just a few bawls and possibly a complex finger board if I can make it or if not then just a few finger ledges, set as an overhang with the side panels also being utilised
If you can choose what size to make it, sheets of ply come at 1220 x 2440 mm, or 1.2 x 1.4 meters, so to make the most economical use of your material you could make the structure as wide as two or three sheets, saves on cutting and waste.
Is it a free standing brick wall, or the side of a house?
4"x2" reminds me of that harry and Paul sketch https://m.youtube.com/watch?v=T1UHwZS3FE8
I'm not too sure on size because I'm not at home at the moment, it will probably be attached to brick work and might only be limited to a small section of just a few bawls and possibly a complex finger board if I can make it or if not then just a few finger ledges, set as an overhang with the side panels also being utilised