Hi @trolleyman,
Sure, I will be happy to assist you in resolving your popup window size issue.
I checked and confirmed your popup.
See here: https://share.zight.com/P8uDQDq4
To set the default size of the popup window, go to your on-page Popup > Section Settings > Design > Sizing. From there, you can adjust the Width, Max Width, Min Height, Height, and Max Height to meet your specific requirements.
I have demonstrated this in the video below. It will assist you in configuring the height and width of your popup to match your custom size.
See here: https://share.getcloudapp.com/nOuLNKAp
To set the default size of the popup spacing, go to your on-page Popup > Section Settings > Design > Spacing. From there, you can adjust the Margin and Padding to meet your specific requirements.
See here: https://share.zight.com/NQu5lBoz
I hope this information helps. Please do not hesitate to ask any further questions. I am happy to assist you. Thank you.