In every culture there are gods, And these gods all have a said appearance. This appearance is, I think, based on the beliefs and Ideas of the religion. What I'm asking for is a decent, detailed answer for what goes into the theological appearance of a god.
Most religions (but not all) start from a figure similar to the followers of the religion - i.e. human like.
They then make them larger, to suggest more power and presence.
They often then add animal or other non-human features (A Jackal's head, extra arms, etc) in order to emphasize that they are not humans but a completely different order of beings.
But really this can go in any direction. They may be smaller (fey), larger (titans), abstract (holy spirit), animal spirits, etc. So you have a lot of freedom.
Start from the religion itself, work out what that religion wants. Do they want a fearsome god to make their enemies tremble? Then think of ways to achieve that. Do they want a god of peace and love? Then how do you achieve that?
Adding to Tim's answer,
When you have a single god, the expectation is the god looks similar to the worshipers, mostly larger and awe inspiring.
However, when you get a pantheon then their appearance generally starts to take on characteristics of what they are the god of. A god of black smiths will likely be a huge muscled man, maybe with the head of a bull since bulls are often a sign of strength or virility.
The gods of alcohol are often jolly partyiers. The gods of death take on aspects that the culture associates with death. Fertility is almost always a woman. etc.