I've just had my first flights on AA (145+763) with a new Limbo 56x45x25, which is essentially the same size of my previous rollaboard that I've used for years on sCO.

I'd say that in the US and flying F you'd be fine with the larger one. When it doesn't fit, it means you're likely on an ERJ, CRJ or Q400 and you can gate check it - although I must say I can't quite remember how it works on the larger ERJ-170/190.