The ABV of Martini is 15%. This brings it into the wine classification. The measure should be 50ml, and this is what you will get in any honest establishment. But many pubs are too mean and will try to con you into accepting less. If they try this, protest, and in future drink elsewhere.
Martini and similar strength drinks should be sold at twice the measure of the strong spirits, ie. 50ml in modern day terms. However some establishments serve some low abv drinks in 25ml measures if the 'bottle price' is expensive. - check the price list dispalyed and point it out if you are not sure.. most bar staff these days wouldn't be able to give a reason and probably double the measure anyway! (wishful thinking)...