diff --git a/variables.tf b/variables.tf
index a6f5bff..b4117f4 100644
--- a/variables.tf
+++ b/variables.tf
@@ -3,6 +3,21 @@
 variable "instance_size" {
   description = "Size of instance to create"
   default = "t2.micro"
+  validation {
+          condition     = ( 
+            var.instance_size == "t2.micro"  ||
+            var.instance_size == "t3.medium"  ||
+            var.instance_size == "m5.large"   ||
+            var.instance_size == "c5.large"   ||
+            var.instance_size == "c5a.large"  ||
+            var.instance_size == "m5.2xlarge" ||
+            var.instance_size == "c5.2xlarge" ||
+            var.instance_size == "m5.4xlarge" ||
+            var.instance_size == "c5.4xlarge" ||
+            var.instance_size == "m6i.large"
+          )
+          error_message = "Input instance_size must be set to an approved vm instance type."
+      }
 }
 
 variable "vpc_cidr" {