diff --git a/NEWS.md b/NEWS.md
index 823adbcfa8..8123476047 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -324,6 +324,8 @@
   (@teunbrand, #3669). 
 * Added `scale_{x/y}_time(date_breaks, date_minor_breaks, date_labels)` 
   (@teunbrand, #4335).
+* (internal) `legend.key.width` and `legend.key.height` calculations are no
+  longer precomputed before guides are drawn (@teunbrand, #6339)
 * `ggsave()` can write a multi-page pdf file when provided with a list of plots 
   (@teunbrand, #5093).
 * (internal) When `validate_subclass()` fails to find a class directly, it tries 
diff --git a/R/guide-colorbar.R b/R/guide-colorbar.R
index 287b0087b8..5116e1b24b 100644
--- a/R/guide-colorbar.R
+++ b/R/guide-colorbar.R
@@ -293,10 +293,10 @@ GuideColourbar <- ggproto(
     # We set the defaults in `theme` so that the `params$theme` can still
     # overrule defaults given here
     if (params$direction == "horizontal") {
-      theme$legend.key.width  <- theme$legend.key.width * 5
+      theme$legend.key.width <- rel(5)
       valid_position <- c("bottom", "top")
     } else {
-      theme$legend.key.height <- theme$legend.key.height * 5
+      theme$legend.key.height <- rel(5)
       valid_position <- c("right", "left")
     }
 
diff --git a/R/guides-.R b/R/guides-.R
index 83ced80cd7..b6da9f723d 100644
--- a/R/guides-.R
+++ b/R/guides-.R
@@ -491,10 +491,6 @@ Guides <- ggproto(
       character(1), USE.NAMES = FALSE
     )
 
-    # Populate key sizes
-    theme$legend.key.width  <- calc_element("legend.key.width",  theme)
-    theme$legend.key.height <- calc_element("legend.key.height", theme)
-
     grobs <- self$draw(theme, positions, theme$legend.direction)
     keep <- !vapply(grobs, is.zero, logical(1), USE.NAMES = FALSE)
     grobs <- grobs[keep]