I was also thinking it'll have to be a big ship. Obvioulsy much larger than a SD because they'd have to include loads of warp cores and fusion reactors for power. I'd estimate between 5 - 10 kilometers.
Remember, I say all tech that Starfleet has had contact with.
That includes:
Transphasics
Batmobile armour
Trilithium Weapons
Genesis Weapons
Phase cloaks/regular cloaks
Etc
I am not sure how many warp cores and reactors such a vessel would need to match a SD's shield output and weapons, so i'm relying on your math abilities.
This vessel would be a total war machine, no facilities for families etc. Every scrap of space is used for power and weapons. (with life support in there somewhere)

Soooo, your opinion please. Would the 24th century Federation be able to construct a SD rival? Remember, they are concentrating ALL their resources on this.