Learn Danish: Bankrådgiveren sagde at overtrækket er for stort. - The bank advisor said that the overdraft is too large.

In indirect speech, Danish often keeps the tense of the original statement if it is still true at the present moment. Since the overdraft is still too big right now, er is used instead of var.

Why does the adjective stor have a -t added to the end here?

Adjectives must agree in gender with the noun they describe. Because overtrækket is a neuter noun (an et word), the adjective stor takes the neuter ending -t to become stort.

I see at introduces the clause, but shouldn't there be a comma right before it?

It depends on which comma rules you follow. Traditional Danish comma rules require a comma before subordinate clauses. However, the alternative official system allows you to drop this comma before at. Both sagde at and sagde, at are completely correct.

How is the word overtrækket built?

It is a compound of over and træk (a pull or draw), plus the definite neuter suffix -et. It literally translates to the over-draw, which perfectly matches the English concept of an overdraft.