Easy house rule. Enlarge increases you one size. Reduce lowers you one size.
You get the STR and CON boost, DEX penalty, AC and attack penalty, Hide penalty, increased carrying capacity, grapple bonus... i.e., most of the basic things in the Monster Manual rules for advancing a size.
For balance purposes though, you shouldn't give Enlarged players any extra reach, faster movement, or Natural Armor. That'd be WAY too strong for a spell of that level.
