Neither component's reference doc said anything about size. Measured in an 896px-tall viewport: the dialog panel is content-sized and stops at 90% of the dialog's own box, after which the body is the only scroll region — the header and footer slots keep their place however long the content grows.

The tab panel has no height of its own: without one it grows with the active panel, with one the content area becomes the scroll region and the tab bar stays put. The bar scrolls on its own axis when the tabs overflow it, so a long tab list never pushes the content out.

Also documents the size lever that was only visible in the source: there is no size property, but a height on the dialog host lowers what the 90% is measured against. It only lowers the maximum — it does not reserve a panel of that size. Browser regressions pin all of it.