The really big ones? We could build something large, technologically speaking, but we couldn't boost something like that out of atmo. So we'd have to spend ludicrous resources launching construction equipment to build the construction equipment to build something that large and then boost up all the materials to do so. The cost would be astronomic and then....what? We don't have the propulsion tech to go much of anywhere with it.

If you mean the actual full ship rather than just the size....honestly they're packed with tech we don't have and I'm not sure where to start.