Each species has a stage the buyer wants. Button: a firm cap of 4 to 5 cm with the veil still closed in DMR's manual, 3 to 4 cm in the farmer folder. Milky: a cap of 5 to 6 cm in the folder, 7 to 8 cm in the manual. Paddy straw: at the button or egg stage before the volva breaks, picked two or three times a day. Shiitake: early, broken from the block by the stalk, never torn.
Oyster is picked before the cap edge flattens and starts to curl up; the site guide treats that as the sign, since DMR gives no cap size.