To be honest, I can't say I've ever had this problem... However, on the revamp, there will be an option to have much larger emoticons (won't spoil anything else about it though). As for the actual size of them now, I could probably create some CSS to increase the size for you a bit. Although, the pic you posted doesn't seem to have larger emoticons compared to ours... they're just yellow.
Just finished my CSS hack:
img.mceSmilie {width: 22px;}
I can't guarantee the quality of upscaling it though. For that, @Optic would probably have to help you out.