A dime bag of drugs always means $10 worth no matter what it is. Yes, bag size can change. In the early 70s a dime bag of weed was a 1/4 ounce bag. That would be about 1/4 of a ziplock sandwich bag.
A dime bag of marijuana is pretty common — hence for instance “Dimebag Darrell” the moniker of the guitarist for the metal band Pantera — but my impression is that a “dime bag” of powder cocaine almost isn’t a thing since $10 is such a small amount, like almost too little to bother with I would think.
It makes me wonder if whoever said the bag is “dime sized” actually meant that it’s the size of a typical dime bag of marijuana. Obviously the substance is cocaine but maybe they were using a dime bag of marijuana as a size reference.
In the picture that was floating around when the story first broke, the bag looked like it was a fair size, not some tiny thing.