I am finding the field sizes for the derby every season unrealistically large at the moment having 20 + runners. I Know some years you will get a large field size but it would be good if some years the amount of runners where less but a quality field.

Yeah, the maximum field size for the (Epsom) Derby in real life is now 20, so it shouldn't really be more than that. I think in the default data files it's set to have a "big" field, but "medium" is probably more realistic.