I'm making my app in Xcode (as an app playground). I understand that the app will be run in a simulator when being evaluated if I choose the Xcode option on the submission form. However, my app looks significantly better on iPhones than on iPads. Is there a way for me to specify which device to use that will be respected by judges? What about device orientation? Thanks.
Can we specify a simulator to be used for our app's evaluation?
No.
All apps are expected to function correctly on every device, because customers can choose which device to run their apps on.
A user can download an iPhone app and have it run in compatibility mode on an iPad, so you have to ensure that it functions correctly on all devices.
By stating that your app looks "significantly better on iPhones than iPads" you are admitting that you haven't written it to function properly when it's run on an iPad. Apps like that tend to get rejected, so spend some time to fix the issues.