SB and PC beat me to the punch. Couldn't disagree more with the comment about using a dead bat. To be a truly effective slapper, you need to master the soft slap, hard slap, and power slap (or swinging away for power would serve the same purpose). The soft slap is often the main weapon and is based on placement as well as hitting it soft. However, proper technique will enable the slapper to deaden the ball even with a "hot" bat. Using a dead bat would prohibit the slapper from hitting hard smashes or high choppers over drawn-in infielders (not to mention gap shots in the OF). This is a must to keep the D honest. You can be the best soft slapper in the world, but a good D will adjust and get you out even if you run a 2.6. Slapping really becomes a deadly weapon when the hitter can decide (often pitch to pitch) where to hit it and how hard. An efficient slapper "messes with" the defense by taking what they are given. Dozens of times I have seen a certain slapper (my DD) go 4 for 4 with a soft slap to 5-6 hole, then with the left side drawn in, a hot smash by 3B before she can even react, then a bunt single (usually with no throw) after 3B soiled herself the last AB. The 4th AB is then usually a "pick em" b/c the D has no clue what to do. Of course, to really mess with them a power slap into the RCF gap for 3 or maybe 4 bags would be the ideal topper after the OF moves way in.
As for the original question, a couple pieces of advice. (1) balanced, not end-weighted, and (2) more length is great but don't sacrifice bat control b/c that is critical in the short game. DD went from a 32 to 33 this past year (Jr year HS, 18u summer). The extra inch definitely helped her reach/ waste a few of those nasty outside drops or change-ups. Make sure your kid can handle the bat though--bat control is probably #2 in importance for slappers (running speed is probably #1).