The larger a molecule is, the higher its boiling point will be
This is because the molecules are held together by intermolecular forces.
The larger a molecule is, the more intermolecular forces there will be and so more energy is needed to break them up
The longer a hydrocarbon chain is, the more intermolecular forces there will be and hence the more viscous the substance will be
The lower a molecules boiling point is, the easier it will be to ignite. Hence, shorter chain hydrocarbons are easier to ignite.