This morning I've build for the first time my game I was working on (prior to the Jam) and it size was 224MB. As the game was nothing special I found it rather big. After compressing and work on the texture the build is now 104MB ( with 35MB of user assets).  Mostlikely due to some third party plugins. 

I'm aware that paid assets /plugins can't be used but to avoid issues uploading in the Jam, is there a size limit?

There is no size limit for the jam itself. Itch has a size limit of 1GB which can be raised by request. https://itch.io/docs/creators/faq#what-limits-apply-to-projects-and-accounts

Also, keep in mind that larger downloads may dissuade other people from trying your game.

To add to what @Satinel alluded to with getting people to play your game, if your game works as a WebGL build, allowing people to play in their browser will generally result in more people trying it out since it makes that process simpler.  Having a smaller download size is the next best thing.  Of course, if you have a 750MB download and the game is exceptional and generates glowing comments and word of mouth, it'll get good attention anyway.

As a nerd, I have to point out how good an exercise it is to learn to optimize the assets used and the size of the build, so good for you for delving into that ahead of time.  Especially if you ever intend to develop for mobile, build size can be a really big deal.

Also, if your game is download-only please don't forget to provide versions for non-Windows platforms as well!