I think the "go down a size for a cross bike" theory applied back when all bikes had level top tubes.
Personally, I'd go for a similar size to your existing bikes with a shorter stem, mine is 20 mm shorter, and have the handlebars slightly higher, i.e. an extra 10 mm spacer or two. Bottom bracket drop varies on cross bikes, but US designed and/or manufactured bikes typically tend to have a lower drop than European ones.
I think the "go down a size for a cross bike" theory applied back when all bikes had level top tubes.
Personally, I'd go for a similar size to your existing bikes with a shorter stem, mine is 20 mm shorter, and have the handlebars slightly higher, i.e. an extra 10 mm spacer or two. Bottom bracket drop varies on cross bikes, but US designed and/or manufactured bikes typically tend to have a lower drop than European ones.