For Helium, with only two electrons, the attraction is very small, He can only be liquefied at 4.2 K. For Alkanes, the London forces increase with increasing molecular weight. Methane is small with correspondingly weak London forces holding the molecules together. This means that T fusion and T vaporization will be low, since only a small input of heat is required to overcome the intermolecular forces keeping the molecules together in the solid and liquid phases. The large alkanes have relatively large net London forces and so, can exist as liquids or solids at room temperature. It's not only the size of the molecule, but the shape as well, that determines the strength of the London forces.
Dispersion forces depend on how closely the molecules can approach each other.
Let's compare the boiling points of three structural isomers of pentane. The strength of the London dispersion forces depends on how closely the molecules can approach each other, which is determined by molecular geometry. The chemical reactivity of Alkanes is fairly limited. Basically, alkanes undergo combustion and substitution reactions with halogens. Let's look at combustion first.