Canadian coffee chain Tim Hortons has developed its own vernacular, including the "double-double." The expression is so common it was added to the Oxford Canadian Dictionary.
A double-double is a coffee with two creams and two sugars added. The actual volume of sugar and cream added varies depending on the size of the coffee you order.
A medium double-double contains around 3.5 teaspoons of sugar, and a large one has 6 teaspoons. So, it's proportionate, and a small double-double tastes the same as a large one.
It's hard to say if the term "double-double" will be well-received in Tim Horton locations outside Canada since it's fundamentally regarded as a pretty Canadian turn of phrase.
Similarly, "double-double" may not always be understood in French-speaking Canada. It may be better to use the French phrase "deux-deux" ("two-two") instead.