>>Skylark of Valeron is just large enough to contain the 1000 km
>>diameter circles of a Sixth Order projector.
>
>That's the size of the core. There's Eris-only-knows how many km of
>armour wrapped around it, and the size changes depending on how much
>of a beating it's been taking recently.
>
>My understanding is that it's supposed to be the size of a large moon,
>so it's probably somewhere around 2-5Mm in total.

Well, Seaton himself says:

"She's just as small as she can be and still do the work....we built the Skylark of Valeron just large enough to contain those thousand-kilometer circles."

Besides, with the kind of forces these people were employing, any merely physical armor, no matter its thickness or strength, would be meaningless.