set default A4

src/main/java/stirling/software/SPDF/model/api/filter/PageSizeRequest.java

@@ -14,6 +14,7 @@ public class PageSizeRequest extends PDFComparison {
     @Schema(
             description = "Standard Page Size",
             allowableValues = {"A0", "A1", "A2", "A3", "A4", "A5", "A6", "LETTER", "LEGAL"},
+            defaultValue = "A4",
             requiredMode = Schema.RequiredMode.REQUIRED)
     private String standardPageSize;
 }