I was wondering if there are any unspoken rules about this/whether anyone has any thoughts? I'm inclined to go with 11pt TNR 1.5 spacing, which gives me something like a reasonable word count to work with. I have seen a couple of agents say to send 'material' double spaced. Obviously the manuscript should be, but surely for a one page synopsis this would end up being too short to cover the salient points of most stories in a way that is actually useful to the reader?
It should be 12pt TNR or similar easy-to-read size and font, not 11pt. My editor at Harlequin accepts single spacing for the synopsis. The manuscript itself is double spaced. You should be able to get the main points into a 1-2 page synopsis. If you are writing more you are giving more detail than they need. Also, NEVER waste words with quotations from the work. I also have about half a page with basic points on setting, main theme, and character breakdown as well but not every editor wants this...in the US maybe you add it to the cover letter, I don't know. I've only submitted in the UK and the cover letter tends to be more about your experience and motivation, not the work.
your synopsis should be TNR 12 pt, single spacing. if you shoot for less than 600 words you'll make the page limit. Good luck!