The answer is OPT, three letters. Short slots take it nearly every time, and few three letter entries get more work in American puzzles. Four squares want RULE, five want ELECT, six split between SETTLE and CHOOSE, and seven take RESOLVE. Count the blanks first, because this clue has no single default across every length.
Opting, ruling and settling are three different decisions
English gives you a separate verb for each way of deciding, and the grid exploits all of them. To opt is to pick from options already on the table. To rule is to decide as an authority does, which is why a judge rules and a spectator does not. To settle is to end a dispute that was open, so it carries the idea of something unresolved beforehand. To resolve is to decide internally, in your own head, about what you are going to do. To elect, in this sense, is simply to choose, and it carries no ballot with it despite appearances.
A bare Decide clue leaves every one of those in play, but a clue with even one extra word usually tips the balance. Decide, as a judge is RULE. Decide, in a way is often OPT. Decide finally is SETTLE.
ELECT is the one that trips people up, because the ballot sense is so much louder than the older one. In a clue about deciding it means simply to choose, as in she elected to take the later train, and no vote is involved anywhere. Editors like it precisely because the word looks political and is not, so treat five squares under a deciding clue as a live candidate for ELECT even when nothing in the puzzle suggests an election.

Counting the blanks before you commit
| Letters | Answer | Flavor of the clue |
|---|---|---|
| 3 | OPT | Choosing between set options |
| 4 | RULE | Deciding with authority |
| 5 | ELECT | Choose, without any ballot involved |
| 5 | JUDGE | Weigh and then decide |
| 6 | SETTLE | End something that was open |
| 6 | CHOOSE | The plainest reading |
| 7 | RESOLVE | Make up one’s own mind |
| 9 | DETERMINE | Long entries, formal register |
OPT is one of the grid’s true workhorses
Three letters, all of them common, arranged in a pattern that will sit happily almost anywhere: that is the profile of an entry a constructor uses to rescue a corner. The word also comes with a full set of variants, so OPTS, OPTED and OPTING can each fill a different slot, and the two phrasal forms OPTIN and OPTOUT give editors modern clue angles about mailing lists and retirement plans. Once you have seen how often the letters appear, a three letter slot under any deciding or choosing clue becomes close to automatic.
It is worth knowing the competition for those three squares anyway. SAY fits a clue about having the final word, as in who gets to decide. SET fits wording about fixing a date or a price. FIX itself works when the clue means to arrange something in advance. None of them are synonyms of OPT, but all three answer a clue that mentions deciding, so a crossing letter in the first square usually sorts them out in one move.
The preposition in the clue is not decoration
Small words attached to Decide are doing real work, and ignoring them is the most common way this clue goes wrong.
- Decide on usually wants OPTFOR, NAME or PICK, since the choice lands on a specific thing.
- Decide against points at VETO or NIX, which are rejections rather than selections.
- Decide to leave, or similar wording about withdrawing, wants OPTOUT.
- Decide to join wants OPTIN, and modern puzzles use it often.
Tense and number have to agree
Crossword clues and their answers match grammatically, and that convention is reliable enough to use as a tool. Decided in the past tense means the entry ends in D or ED, so OPTED and RULED become live and OPT does not. Decides in the third person means the entry takes an S, so OPTS and RULES are in play. Deciding means an ING ending. When you have a length that seems one letter too long for the obvious word, check the tense of the clue before you abandon the word, because that extra square is very often nothing more than a grammatical ending.