OK writers, should I double space my story?

The publication that does not specify a style they want for submission expects you to use the standard format.

Courier 12 or Times New Roman 12
One-inch margins on all four sides
Double line spacing
Indent paragraphs 1/2 inch using automatic indentation, not the tab, not spaces
No extra blank lines between paragraphs
Underline text which would be italicized in the printed book in Courier, italicize in TNR
Ragged right margin (not justified)
Turn off hyphenation
Turn off widow and orphan protection
Slugline, upper right corner, on pp 2 - end: Surname/ONEWORDTITLE/# (example: Dickens/CITIES/75)
Start page 1 1/3 of the way down the page
At the top of page 1, your name, address, contact into upper left corner, word count rounded to closest 100 (for short fiction) and rights offered for sale in upper right, flush to margin

There is a special format that is used for books. You can see what it looks like by looking at published books and they are single spaced . You need a good word processor to achieve the proper format.

If you contact them, they might say "spacing doesn't matter". Go with single spacing. If they don't use your story, a matter for which there might be several possible reasons, try double spacing, next time.

Contact the publication and ask directly.