The general requirements are (and @User will need to correct me if I have any of this wrong) that you will need to have at least GDS of your project and have it up in a public repository like github; you will want to fork the efabless/caravel project from github and replace the example project in it with your user project. The github repository for caravel does not yet have the layout; we are still running a few passes through openlane. If you can keep your project within about 2.8 x 2.8mm, it should be okay; I believe we are on target to get closer to the 10mm^2 we promised, which is more like 2.8 x 3.2 or maybe a little taller.