I am following a recipe that mentions 4tbsp of coriander leaves and 5tbsp for Mint. How can one measure leaves in the weight of tbsp? 
It refers to spearmint in this case. You can find both spearmint (Mentha spicata, known in Greek as δυόσμος) and peppermint (Mentha × piperita, generally known as μέντα, though that may be used for almost any type of mint) pretty readily throughout most of Greece, but spearmint is much more common in traditional Greek cuisine (peppermint is generally too strong for how they typically use mint), and all the original Greek recipes I’ve seen for tzatziki that call for mint call for spearmint. You can, however, substitute most other mildly flavored mints and get a similar flavor. I’ve used both Mentha arvensis (commonly known as field mint or wild mint) and Menth suaveolens (commonly known as apple mint) before in my own home made tzatziki and the difference was not readily noticeable. 