Nope, no such thing as too big, but how it is incorporated into a piece will affect how "flashy" it looks. Personally would love bouquet styled brooch with a mix of 5-10ct, candy-esque colored diamonds :
: and would wear that bling on everything and anything. To be honest, with my casual wardrobe amd casual lifestyle, most bypassers would assume its paste or Swarovski (fine by me!) and not give it second thought. If it could pass as costume jewelry in context, then that will be most likely assumption.
On the flip side, a single 5ct stone set into an engagement ring type setting catches much more attention, imho due to the general assumption that an engagement ring is not a costume piece...even if it is!