How large could a planet be yet still approach 1 Earth gravity and support life as we know it?
Life on Earth has developed with one Earth-gravity, more or less, as a constant for epochs. Our bones and organs both are adapted to this amount of pull, (or push, depending on how sciency you want to put it, right?) but could a larger planet have a similar pull (at the surface) based on various factors such as less planetary density, slower planetary rotation, or a counter-pull from another structure, like being surrounded by a dense shell?
Based on this conception that it's possible to explain a larger planet with one Earth gravity at the surface supporting life as on our planet, how large could that other planet be, and how could that happen?
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For a spherically symmetric planet, surface gravitation is determined by just two quantities: The average density, $\rho$, and the radius, $R$. In particular, due to spherical symmetry you can consider the whole planet's mass to be concentrated in the center, and then you get for the gravitational acceleration at surface:
$g = G\frac{M}{R^2}$ with $M = \rho V = \frac{4\pi}{3}\rho R^3$
and therefore
$g = \frac{4\pi G}{3}\rho R$.
This means if you want the same gravitation on the surface for a larger planet, you need to reduce its average density by the same factor.
The problem is that you cannot arbitrary reduce the density, as sufficiently away from the surface, the pressure will make sure your material is compressed to have no significant holes, and therefore it will only the substance itself that determines the density.
One possibility is to not have an iron core. However that would in turn mean there's no magnetic field to protect you from cosmic radiation and star wind. According to this page, the core (inner and outer combined) has a radius of 3400 km (2100 + 1300), and makes up 32% (or roughly 1/3) of the earth mass. With an earth radius of about 6400 km, it is slightly more than half the earth radius, which means it has a bit more than 1/8 of the earth's volume. Thus replacing it with mantle material would reduce earth's density by roughly 25% (actually less because the core is under larger pressure and therefore denser). Assuming this density doesn't change significantly when making the planet larger, this would mean you would end up with a planet whose radius is about 4/3 the earth's radius (and the surface is therefore 16/9 the earth's surface, or 77% more than earth's surface).
Since the mantle is also denser than the crust, I think another density reduction (and thus radius increase) should be possible (but maybe at the cost of continental drift/geologic activity; those might have played a major role in the emergence of life).
Note that a spherical shell around the planet would not affect the gravitation on the surface at all, since such a shell doesn't cause a gravitational force in the inside.
A fast rotation would reduce the apparent gravitation near the equator, but any rotation fast enough to make a noticeable difference would also make the Coriolis force quite strong. Also, it would do nothing for the polar regions, so you'd get a steep gravity gradient over latitude. I'm not sure that would be very life friendly.
A "counter-pull" from another structure is known as tidal force. I'm pretty sure that before it would make a noticeable difference on the gravitational pull (note that you don't feel the tidal force of the moon on Earth, you only see it from the tides) the tidal forces would tear the planet apart.
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