I agree with the above, for drawing blood in small animals we typically use a 18-20ga needle. RBC's are typically 6-8um in diameter, while typical yeast can range from 2-40um, although I am not sure what the range is for brewing yeast specifically.
Of course, you can always go with the rule of thumb that the bigger the needle, the easier it is to draw, so go big or go home. Since you are not worried about sticking it in a critter, you could easily go with a 10-12-14ga needle.
A tip on looking for needles of that size, you may be able to find a veterinarian that does horse or cattle work, they will have needles of that size for sure.