diff --git a/arch/sim/src/sim/up_internal.h b/arch/sim/src/sim/up_internal.h
index 0a69ca97188..f40de3574e7 100644
--- a/arch/sim/src/sim/up_internal.h
+++ b/arch/sim/src/sim/up_internal.h
@@ -306,7 +306,7 @@ unsigned long up_getwalltime(void);
 
 #ifdef CONFIG_SIM_X11FB
 int up_x11initialize(unsigned short width, unsigned short height,
-                     void **fbmem, unsigned int *fblen, unsigned char *bpp,
+                     void **fbmem, size_t *fblen, unsigned char *bpp,
                      unsigned short *stride);
 #ifdef CONFIG_FB_CMAP
 int up_x11cmap(unsigned short first, unsigned short len,
diff --git a/arch/sim/src/sim/up_x11framebuffer.c b/arch/sim/src/sim/up_x11framebuffer.c
index f3634accc35..94cd92248fb 100644
--- a/arch/sim/src/sim/up_x11framebuffer.c
+++ b/arch/sim/src/sim/up_x11framebuffer.c
@@ -359,7 +359,7 @@ shmerror:
  ****************************************************************************/
 
 int up_x11initialize(unsigned short width, unsigned short height,
-                     void **fbmem, unsigned int *fblen, unsigned char *bpp,
+                     void **fbmem, size_t *fblen, unsigned char *bpp,
                      unsigned short *stride)
 {
   XWindowAttributes windowAttributes;