1) Bring up Printer Control Panel ( Start->Settings->Printer and Faxes )

2) Right click on Dell 3115cn printer and select Properties

3) Click "Configuration" tab.

4) Click on Custom Paper Sizes button at bottom.

Note: It appears you need admin privilidges to set custom paper sizes.

I haven't tried doing this myself... just happen to discover where the settings are.  Reply here if it works.

Answer for Windows 7

Go to Control Panel -> Printers and Devices.

Select your printer.

Choose (translated): Properties of Print Server

I ONLY see this option from the list with common tasks shown on the top of the screen.

It begins with (translated):

Add device // Add Printer // View Print Jobs // Manage Default Printer / Properties of Print Server

Choosing Properties of Print Server, brings a follow up dialogue. On the bottom you have a button to change Form Settings.

Don't forget to save.

Thanx : )

All 20 custom paper sizes are grayed out.

I'll repost if I find an answer.

I created my own template, but that did not show up in the print dialogue and its settings.

Solution is to 'just' change one of the 20 editable formats they offer.