To convert from instant dry yeast (IDY) to fresh yeast, you triple the amount of IDY, by weight (not by volume), to get to the required weight of fresh yeast to use. (To convert from ADY to fresh yeast, you double the weight of the ADY.)
If you are using one of those little cubes of fresh yeast from the supermarket, like Fleischmann's fresh yeast, the cube weighs 0.6 oz (17 grams). So, for example, if you used 0.03 ounces of IDY (which is typical for a single 16" Lehmann dough), tripling that would give you 0.09 ounces of fresh yeast on a comparable basis. That would come to 0.09/0.6 = 0.15, or a little over 1/6th of one of those little cubes of fresh yeast.
I hope you will let us know if you detect a difference when using the fresh yeast in lieu of the IDY. Remember, there is no need to hydrate the fresh yeast in water. It can be so hydrated if you prefer (in tepid water only), but it can just be crumbled directly in with the flour, just as you now combine IDY with the flour.