In summary: in higher gravity, humanoids must be smaller or they won't be able to support their own weight. However, smaller creatures cannot have complex enough nervous systems to be intelligent.
The first part of the answer involves the gravity of a planet. According to Newton's law of Gravitation the gravitational force on a object is:
$$
F=G\frac{m_1 m_2}{r^2}
$$
Ignoring the factor of $G$, the gravitational acceleration (force over mass) is just proportional to the mass of the planet $M$ and the radius of the planet $R$ (for an object at the surface, distance is equal to radius):
$$
g\propto \frac M{R^2}
$$
The mass of a planet, assuming the density remains constant, is proportional to volume, which is proportional to radius cubed:
$$
M=\rho V\propto R^3
$$
So we find that the surface gravity of a planet is proportional to the radius:
$$
g\propto\frac{R^3}{R^2}\propto R
$$
So if you double the size of the planet, the gravity also doubles.
The second part of the question involves living organisms. The shape of an organism is related to its size due to something we call the square-cube law. Essentially, the strength is proportional to area (size squared) but weight is proportional to volume (size cubed). This is why a person could not simply be scaled to gigantic size, they would need to become thicker (as you pointed out).
As the ratio of weight to strength changes, structures must become change shape. As the ratio increases (more weight) structures must become thicker (e.g. an elephant); as the ratio decreases (less weight) structures can become more spindly (e.g. a fly). Note that most specific shapes occur at a specific range of this ratio, i.e. a specific size: there are no upright-walking humanoids much larger or smaller than humans.
For a given density, the weight is also proportional to gravity, so the ratio is proportional to gravity and size:
$$
\frac WF\propto \frac{gl^3}{l^2}\propto gl
$$
This means that to maintain the same ratio, and therefore the same (humanoid) shape, a creature must become smaller as the gravity increases. Therefore, the size of creature that will evolve a humanoid shape is inversely proportional to planet size. So on a planet twice the size of the Earth, you would have 1 m/3 ft humanoids.
There are size limits: smaller humanoids may not have complex enough brains to be sentient, so the gravity can't increase without limit. If you regard children or dwarves as fairly close to the minimum size/complexity, then we're looking for around 1 m/3 ft creatures.
At around half the size of average humans, a 1 m/3 ft humanoid could evolve on a double gravity/double size planet. This is a very, very rough estimate, so the absolute limit could be higher (maybe 4 or 5 g).