There's a theory that the reason Mercury has such an enormous iron core is that it was once a much larger planet before it got impacted by an object, resulting in most of its mass getting blasted away. If this theory is true, how large was Mercury before the impact? Could it have been the size of Earth or Mars, or maybe even a super-earth?
If we look at the planet's cores and I'm going to ignore liquid vs solid and focus on size overall.
Mars: Core estimated 1,794 +/- 65 km radius. The planet is 3,390 km radius. About 53% of the planet's radius is its core. Mars also has more sulfur in it's core and more Iron in it's mantle than Earth, suggesting that it probably didn't mix as well as Earth did, but I'm not sure that would significantly effect the size of it's core.
Venus: Core estimates have some uncertainty, but by this article, it's core is thought to be about 3,000 km, about 49.5% of it's 6,052 km radius.
Earth's core is about 3,400 km and it's radius 6,371 km, about 53.4% of it's total radius. If we use the 49.5%-53.4% as a guideline, Mercury's 2,440 km radius (85% core, so it's core is about 2,074 KM), so a rough estimate using the other 3 planets as a guideline, 3,880 - 4,190 km radius. That puts it 500-800 km larger than Mars in radius, roughly 2,000 km smaller than Venus in radius.
That's obviously just a rough estimate, but assuming it was similar to the other 3 inner planets, it's probably in that range. I should probably also account for compression. The more massive the planet the greater the compression of it's core, but that would probably only vary a couple percentage points off the estimate and it wouldn't make a big difference.