how do you show a custom resource tag on the resourcebubble event

The resource bubble receive a simplified resource object with basic information only. However, you can use it to load the full DayPilot.Row object like this:

resourceBubble: new DayPilot.Bubble({
  onLoad: args => {
      const r = dp.rows.find(args.source.id);
      const size = r.data.tags?.size;
      args.html = `Size: ${size}`;
  }
})

This assumes the resources specify the size value using the tags property:

const resources = [
  {name: "Room 1", id: "A", tags: {size: 5 }},
  // ...
];
dp.update({resources});