Learn Croatian: Film je bio prilično dug.

Because dug is in the predicate (it comes after the verb to describe the subject). In standard Croatian, when an adjective describes a noun this way, it takes the short (indefinite) form. If you were saying "the long film" directly, you would use the long (definite) form: dugi film.

Doesn't the word dug also mean "debt"?

Yes, it does! Dug can be the masculine noun for "debt" or the masculine adjective for "long". You tell them apart by context. Here, the adverb prilično (quite) comes before it, which only makes sense for an adjective.

Since je is a strict second-position word (clitic), could I drop the subject and just say Bio je prilično dug?

Exactly! If you drop the subject film, the past participle bio becomes the first word. Because the auxiliary verb je must stay in the second position, it slots right in after it: Bio je prilično dug.

Could I use dosta instead of prilično here?

Yes, in everyday spoken Croatian, you will often hear Film je bio dosta dug. Both words can be used to mean "quite" or "fairly" in this context. Prilično is just a slightly more precise way to express "considerably" or "rather," whereas dosta literally means "enough."