I don't normally use plastic "worms" to fish for Crappie, but if that's what your fish are feeding/hitting on ... then they'll likely "hold onto" the salted ones a little longer.
I wouldn't use one over 4" ... and may even cut a 4" one down some, depending on whether or not I was getting hookups or not.
If plastic worms was what worked on the waters I fish, I'd likely go with Charlie Brewer Sliders ... either the 3" Drop-shot worm or the 1.5" Panfish Grub (paddletail). And I'd probably use the Junebug/Chartreuse color in either/both.
If I didn't have to worry about weeds or snags ... I'd put them on a 1/32 - 1/16oz jighead w/#4 hook. But, if weeds/snags were a problem ... I'd use a weedless jighead, or possibly even the CB Slider weedless jighead.
You can see all the Charlie Brewer baits here : http://www.sliderfishing.com/ ... very good baits, good color selection, and they catch fish !!
But, if you're talking about "small" Crappie, like <9" ... you'll probably catch more of them with a 1/32 or 1/16oz marabou Roadrunner in solid Chartreuse or Pink. Many of our bait making members also custom make Roadrunner clones ... and they can custom make them in colors not available from Blakemore.
... cp