I've just had the calculator out.
Pubs used to serve single measures of spirits as 25ml, and many still do, but some now use 35ml measures.
Using the OLD way of measuring, an 8oz flask holds 4½ DOUBLES. Using that as a reference point, you should be able to see whether, say, a 3oz or 4oz flask would be big enough. (It might well be if the function of the flask is simply to be able to tip some booze into a coffee in Costa. It might not be if the purpose of using the flask is to get somewhat 'relaxed'!)