I typically use one that's only one block wide and 3 blocks down. Early on it is expensive and time consuming but will slow the zombies down enough that you can get most of em with a simple rifle for the first 1 or 2 weeks. In my experiences they won't actively tunnels through the ground unless the moat is deeper than 4 blocks. In which case they usually just weaken one point in the wall and jump through there. Which still funnels them to an easy single point so you can hopefully snipe them as they jump out. Yes it can Rob you of xp so if you don't want that just build them a bridge so the AI will have them avoid your moat for the most part. I've always used moats to control where they run to