Susan, A guideline I always worked with is based on a secured, vertically solid load column. It says the stack height should not exceed 6 times the narrowest base dimension. I.E.: If the load is solidly packed cases, interlocking layers, without head space in the cases, and measures 40" x 48", the top of the column should not exceed 240" high.
If the pallet is 48" x 40" GMA, but overhangs the product stored on it, the rule applies to the product dimension, not the pallet dimension.
Common sense adjustments need to be made if the cases or product are not of sufficient columnar strength to support the weight without crush or lean.