Learn Russian: Она долго загорала и теперь красная. - She sunbathed for a long time and is red now.

Because of the word долго (for a long time). Whenever you emphasize the duration of an action, Russian requires the imperfective aspect, even if the action is completely finished and has a visible result. If you removed долго and just wanted to focus on the result (She got a tan), you would use the perfective verb загорела.

Could I use сейчас instead of теперь for the word "now"?

You could, but теперь is the more natural choice here. Теперь specifically highlights a change in circumstances or a new state that is the result of a past action (she wasn't red before, but now she is). Сейчас is a more neutral word simply meaning "at this current moment."

Since being red from the sun is a temporary state, shouldn't we use a short-form adjective here?

That is a logical guess, because short-form adjectives are frequently used for temporary conditions in Russian. However, color adjectives like красная (red) are almost never used in the short form in everyday modern speech, even for temporary situations like a sunburn. You simply use the standard long form to match the feminine subject (Она).

Is красная the standard way to say "sunburned" in Russian?