Short date or past best by dated liquid yeasts. They are perfectly fine to use if you make up a yeast starter prior to pitching, there will just be low viable yeast counts. Size dependant on type of beer you are making but 2 litre starter should suffice for most uses. Dissolve 200g of Spraymalt and make up to 2 litres with water. Boil for 5 mins then cool and put in a demi john. Pitch the yeast, shake and cover loosely with tin foil. Shake everytime you walk past it and allow to ferment for 48 hrs before pitching. This will increase the cell counts to pitchable volumes.