This recipe for poppy seed rolls calls for 1 large yeast. How much yeast am I supposed to use? I usually see recipes measure it in teaspoons, but I can’t guess how much “large” is. I am also assuming this is active dry yeast, maybe it’s not and that’s the reason why the size is weird.
This is short for “1 large cake of yeast.” According to this investigation, cakes of yeast traditionally came in two sizes:
This similar recipe gives the substitution “1 large yeast or 3 envelopes dry yeast.”’
A typical conversion I know of is to use 1/3 as much dry yeast as fresh yeast. If we take James McLeod's answer, then a "large" yeast should be replaced with roughly 18 g of dry yeast (close enough to 3 envelopes á 7 g) and a "small" yeast cake should be replaced by 0.2 ounces dry yeast, or 5.6 g yeast (most recipes will tolerate a full 7 g envelope).
I grew up in the 80s and 90s with 42 g cubes of fresh yeast, if you have a recipe for one of these, use 14 g dry to replace. Sadly, I don't know of any way to make sure whether a recipe refers to the sizes James McLeod's answer uses, or the 42 g size - you will have to take your best guess based on the time and place the recipe was written down.