The only limit is your imagination.
So, remember how in LittleBigPlanet and LittleBigPlanet 2, your level size was limited entirely by the thermometer? Where, if you tried to put too many things into your level, the thermometer would fill up and you’d be unable to put in any more stuff? Well, apparently, that limitation is gone (at least in the PS4 version of the game; keep in mind the game will also be coming to the PS3 after all), meaning the only limit to any custom created level is literally your own imagination (as cliche as that sounds).
In LittleBigPlanet 3, you can stitch multiple levels together, which allows you to bypass the thermometer. This means theoretically, and with some amount of craftiness, you should be able to make levels as long as you want. The stitching functionality itself seems to sound like the link functionality on LittleBigPlanet Vita.
LittleBigPlanet 3 releases on the PS3 and PS4 later this year. Stay tuned to GamingBolt for more coverage.