The written rule is unclear. Still, I vote for "a standard action".
If you allow him to take a full-round action (or a standard action and a move action), very strange thing happens.
Imagine, a sorcerer wants to cast an empowered Fireball. Now this is a full-round action. But not a spell with casting time with "1 round".
He is not surprised and his initiative is 7.
If he doesn't delay, he takes a "Start Full-Round Action" in his turn of the surprise round. Then, take a "Complete Full-Round Actin" in his turn in the 1st regular round, at initiative count of 7. So, the empowered Fireball are shot at initiative count 7 of the 1st-regular round.
Now, he delays, and then choose act in the next round (1st regular round) at initiative count 16. If you allow him to take a full-round action, he can shoot an empowered Fireball now. Thus, at initiative count 16 of the 1st-regular round.
He could shoot the spell faster because he delayed. That is quite strange, isn't it?