[Patrick]

If your using a stock motor mount I think 18t is the smallest pinion you can use without grinding some of the fins off the finned version.
If your talking about the 1700kv 1515 2.5d on 5s, then you shouldn't need to go any smaller than 18t anyway for normal bashing and standard size tires. I'd probably use the 20t mostly.
If your talking about the 2000kv 1512 2.5d on 5s, then 18t should be ok for most stuff if your using standard size tires, but if the sand makes things get too hot you may need a slightly smaller pinion, or the 65t ofna monster pirate spur (needs a few minutes with the dremel first).
Standard 540/550 smooth cans - 36mm diameter
Smooth can neu - 39.6mm
Finned can neu - 43.8mm