Ice spheres are too big to fit in a highball glass, but I always like to use them for my Old Fashioneds and Sazeracs. I have a nice square silicon
mold for four I've been using for a couple of years that's a really good size. My girlfriend also got me a mold for eight this past Yule but they're a bit bigger and awkward to stir in a standard rocks/Old Fashioned glass.
I also recently picked up a few silicon trays to make tiny pebble sized cubes for Mai Tais, so I don't have to break ice in a lewis bag.