How many black tea leaves are required per 125 ml of milk + 25 ml of water?
Loose leaves will be "coarser" than "broken" teas and usually look "rolled up" or "curled". 

I'd start with a bit more (e.g. a heaping teaspoon loose leaf vs. a scant level teaspoon of broken) and see how it turns out. Many sources sugest meassuring tea by weight, thus eliminating the variance due to different "packing" of coarser or finer tea.

The reason why I suggest experimenting is that broken tea vs. whole leaves will have a different flavour profile (even for "the same" tea) due to the loss of aromatic oils and higher extraction of tannins in broken tea.

Wikipedia has an extensive list of tea grades and leaf sizes.

And this website gives volumetric equivalents for the same weight - One heaping tablespoon loose leaf tea as roughly one teaspoon broken tea and a scant teaspoon gunpowder (tightly rolled balls).