Pretty big. If you rearrange Newton's law of gravitation (F=G*m1*m2/r^2) and apply it to surface gravity, assuming the planet is a perfects sphere with a given specific gravity (not gravity-gravity, specific gravity where water = 1.0), then the formula for surface acceleration in Earth G's is:
A (in Earth G's) = 0.1814 * specific_gravity * Earth_radii
The Earth has a specific gravity (density) of 5.513, and of course a radius of 1 Earth radius, so the formula spits out 1.00 G's.
Basalts and granites have a specific gravity of around 2.7 to 3, so if we didn't have the big iron core we could have a planet with half our specific gravity (2.75 instead of 5.5) and twice the radius. That would give us four times the surface area and eight times the volume (area=4*PI*r^2, volume=4/3*pi*r^3), but since we'd be half as dense, we'd only weigh four times as much. If you're willing to use that same light density and let the surface gravity go up to 2 G's, then the radius can double again, multiplying the mass by eight more, so we'd have 32 times Earth's current mass, twice the gravity, and sixteen times as much surface area.
If you're willing to live at 4G's (would fish even notice?) such a planet would have 256 times more mass than Earth, and 64 times as much surface area.