Size of mushroom can be changed with using different hooks, and/or yarn.
Large mushroom is made with Bernat Blanket yarn and K hook. To make one even bigger, try a 9 or 10mm hook.
Medium mushroom is made with Red Heart Super Saver and any hook larger than an F hook.
Small mushroom is made is with Red Heart and F hook or smaller.

Use 20mm or 18mm eyes for large.
Use 15mm eyes for medium.
Use 12mm or smaller for small.