If you're looking for a flat whistle, then I recommend Acme 636. The Acme 636 is not only a great small whistle, it's also my favorite pea-less whistle. I also own Storm, Fox Sharx, Fox Sonik Blast, TOPS, and others. I do not own the ACR whistle. Regarding the Fox Sharx, it is about three times larger than the Acme 636.

The whistle I carry 24/7 is the Acme 660. It is my favorite pea whistle and favorite overall. For size comparison, the Acme 660 is substantially smaller than the Fox Sharx. The Acme 660 is also way easier to blow than any of the whistles I mentioned above. For reference, my daughter was able to make an extremely loud sound with the Acme 660 when she was two years old. She could not do so with any of the other whistles I mentioned.