Expose a few more configurable items and add input validators

variable.size.tf

@@ -1,6 +1,9 @@
 variable "size" {
   description = "The slug indicating the size of the droplet"
-  default     = "s-1vcpu-1gb"
   type        = string
-}
 
+  validation {
+    condition     = can(regex("^[a-zA-Z0-9-]+$", var.size))
+    error_message = "The size must be a valid DigitalOcean size slug (e.g., s-1vcpu-1gb)."
+  }
+}