I'm talking about my experience with other jams, but it should be the same here as well. So:

• There will be a couple of categories for people to rate your game on.
  That is then converted into a single raw score. Note that if your game has less than 10 votes, that score is less than the average, to prevent someone from having a friend give 5 stars to everything and get a perfect score.
• You don't have to post a trailer, or do any sort of advertising. Besides, only Jam participants can actually vote on your game, so you don't really get much out of it. What you can do to increase the number of eyes on you, is play - rate - comment on other games. This way you help someone get a better idea of how their game did, and as a thank-you they check out your game as well.
• I don't think there is a file size limit. At least I've never run into one. and I don't think most games will in such a sort time period. The file size is the same as the one itch.io provides, so you should be fine.

Good luck on your first game jam! Once you get into it it can be really fun!

Note: To submit a game, you need to upload it to your dashboard, publish it, and then come back on the jam page to submit that game as a jam-entry. If you're concerned about fake entries, don't be. The community reports those to the moderators, which then verify and take it down.