For panfish, take a look at a 1XL or (depending on the size of the fly's body) 2XL size 6 or 8 or 10 (again, depending on the size of the fly's body) hooks. Depending on the geometry of the hooks you choose, you may need to upsize the hook a bit to get sufficient gape. The geometries will vary from manufacturer to manufacturer, and sometime even within a single manufacturer's line, but most such hooks should be large enough to accommodate the body as well as hackle and tail.
Regardless of the hook, with poppers, be sure that the body is not too large for the hook you're using and that the body does not sit too low on the hook (if it does, it can interfere with hookups). If you're tying hoppers too you'll also want to make sure that the shank is long enough to allow you to mount the body with enough room remaining for the hackle and tail. When only tying poppers, you can even use standard dry fly hooks if the popper body's not too long.
Either kinked or straight shank hooks can be used. If you use straight shank hooks, simply build a relatively large and rough thread base before mounting the body -- the rough thread base will help the body to get a grip once you glue it to the shank.
Feel free to drop me an email if I can offer further suggestions. Good luck!
The way I cut the foam out my little poppers come out about .326 (between 5/16 and 3/8's) and just shy of 5/8 long. Always trying to figure these hooks out. Make hoppers out of 2mm stuff....probably around 3/8 wide.