From Loren Lassiter
Use a blank wall, preferably in sight of the table, to mark the characters’ relative heights.
It’s a great way for everyone to truly picture how tall their characters are – and how they size up next to their co-adventurers.
We use the wall behind the DM to affix signs that include the characters’ names written in a font and color style that sort of represents their personalities (e.g. gothic lettering and fiery glowing colors for our former demon paladin of Heironious).
This also makes it easy to point at a spot on the wall and say, “your opponent is *that* tall.”