diff --git a/src/compile/mark/encode/position-rect.ts b/src/compile/mark/encode/position-rect.ts
index b2229b1433b..9f88743be9a 100644
--- a/src/compile/mark/encode/position-rect.ts
+++ b/src/compile/mark/encode/position-rect.ts
@@ -153,7 +153,11 @@ function positionAndSize(
 
   // Use size encoding / mark property / config if it exists
   let sizeMixins;
-  if (encoding.size || markDef.size) {
+
+  // TODO(@kanitw): this sizeMixins logic here is a bit convoluted.
+  // It's sometimes set in the defaultSizeRef below and sometimes up here.
+  // We should refactor this logic to be simpler.
+  if (encoding.size || (markDef.size && !isRelativeBandSize(markDef.size))) {
     if (useVlSizeChannel) {
       sizeMixins = nonPosition('size', model, {
         vgChannel: vgSizeChannel,
diff --git a/src/mark.ts b/src/mark.ts
index 77bcd40dc5e..fa2dd8374b4 100644
--- a/src/mark.ts
+++ b/src/mark.ts
@@ -128,7 +128,7 @@ export interface VLOnlyMarkConfig<ES extends ExprRef | SignalRef> extends ColorM
 
 export interface MarkConfig<ES extends ExprRef | SignalRef>
   extends VLOnlyMarkConfig<ES>,
-    MapExcludeValueRefAndReplaceSignalWith<Omit<VgMarkConfig, 'tooltip' | 'fill' | 'stroke'>, ES> {
+    MapExcludeValueRefAndReplaceSignalWith<Omit<VgMarkConfig, 'tooltip' | 'fill' | 'stroke' | 'size'>, ES> {
   // ========== Overriding Vega ==========
 
   /**
@@ -147,7 +147,7 @@ export interface MarkConfig<ES extends ExprRef | SignalRef>
   /**
    * Default size for marks.
    * - For `point`/`circle`/`square`, this represents the pixel area of the marks. Note that this value sets the area of the symbol; the side lengths will increase with the square root of this value.
-   * - For `bar`, this represents the band size of the bar, in pixels.
+   * - For `bar`, this represents the band size of the bar, in pixels, or relative band size (e.g., `{"band": 0.5}` is half of the band).
    * - For `text`, this represents the font size, in pixels.
    *
    * __Default value:__
@@ -158,7 +158,7 @@ export interface MarkConfig<ES extends ExprRef | SignalRef>
    *
    * @minimum 0
    */
-  size?: number | ES; // size works beyond symbol marks in VL
+  size?: number | ES | RelativeBandSize; // Unlike in VG where size is only for symbol marks (point in VL), size works beyond symbol marks in VL
 
   /**
    * X coordinates of the marks, or width of horizontal `"bar"` and `"area"` without specified `x2` or `width`.