diff --git a/THEMES.md b/THEMES.md
index a8a111fa1..a321229fa 100644
--- a/THEMES.md
+++ b/THEMES.md
@@ -445,6 +445,9 @@ Can be created as an extra.
 
 * `pos` - type: NORMALIZED_PAIR.
 * `size` - type: NORMALIZED_PAIR.
+	- If only one axis is specified (and the other is zero), the other will be automatically calculated in accordance with the video's aspect ratio.
+* `maxSize` - type: NORMALIZED_PAIR.
+	- The video will be resized as large as possible so that it fits within this size and maintains its aspect ratio.  Use this instead of `size` when you don't know what kind of video you're using so it doesn't get grossly oversized on one axis (e.g. with a game's video metadata).
 * `origin` - type: NORMALIZED_PAIR.
 	- Where on the image `pos` refers to.  For example, an origin of `0.5 0.5` and a `pos` of `0.5 0.5` would place the image exactly in the middle of the screen.  If the "POSITION" and "SIZE" attributes are themable, "ORIGIN" is implied.
 * `delay` - type: FLOAT.  Default is false.