Upstairs Oriel bow window install- how big can it be?

It’s achievable, an oriel bay is completely different to the 6 seg bays Illustrated. The floors project out with the barrel of the bay. The conversion your asking for would consist of a base . Structurally , and cosmetically supported by a gallos bracket. The head either (slight fall) but classed as flat lead work with mop polling .Or a pitched lead or tiled roof . You could connect both bays by a barrel but this is not the norm ! .It depends on the width of both apertures. In comparison to the projections . Ultimately there’s a happy medium to maintain the character of the building . Kind regards Dean

You are governed by internal plaster walls upstairs and down you can only measure as high as ceilings plus 40mm including add ins and same with internal window boards

You don’t normally find bow windows up stairs because they have to be supported individualy by gallows brackets and they don’t look good high up