Ask her causally, then make her guess why you're asking.

[Alucard's grin shouldn't look so excited at the prospect of making mischief for an old woman, but here they are.]

I'll...try for small. I would very much like nothing more than a ceremony with maybe just two witnesses and nothing else.

[It'll be an impossible argument he will lose.]

But a secret one would be the second best thing, if we cannot avoid even a medium sized gathering.