You can see your account’s current file size limits on the file upload area when editing your project page. By default, any regular account will have a 1GB upload limit per file, and for HTML5 games we have a separate set of limits described here: https://itch.io/docs/creators/html5#zip-file-requirements

As for the theoretical maximum size we support, much much larger, but we tend discourage developers from uploading huge games. Since we host many new and inexperienced developers, it often means signs of unused or un-optimized assets. Instead we ask that you optimize your game to a reasonable size. The larger your game the less likely people are going to want to download it.