By default, SeriousMD already has pre-configured paper sizes for your browser but just in case you want to define your own paper size, here's how you can add your own inside Windows.
1. Just go to Devices and Printers
2. Pick the printer you want and click on Print Server properties at the top (image above)
3. Check the Create a new form checkbox
4. Add the sizes you need
5. Give the Form a name
6. Hit the Save Form button
When you print inside SeriousMD, just pick the papersize that you just created.