how big is too big for engagement ring diamonds/stone?

There is no such thing as "too big" when it comes to engagement rings. It really just comes down to what you love the look of and what suits your style and personality. Remember that this ring will be with you for life, so try to choose a design that you feel you will love forever, but don't be afraid to find something that is uniquely you. 

It really depends on your finger size & your style. Some people like oversize, others prefer small & discreet. You are the only one who should choose this.