I am about at the same stage as you - kit receive, lumber obtained, just waiting on free time to start the build.
I am going to start with a 12’ beam. I won’t quite go 30” above the work surface, but I am trying to fit this under my 7’ garage door. The longer beam will require longer chains. I ordered a 10’ piece of #25 roller chain and links from eBay for around $15.
From what I have read, the longer and higher beam give better accuracy at the bottom corners and top center of the work surface. I don’t want to build a frame and rebuild it later, so starting at 12’.
Also important the beam does not deflect from the weight, so probably best to find a metal beam instead of a 2x4.
There is a very nice spreadsheet with a force calculator that you can experiment with different beam length and position configurations to see what is best. Sorry I don’t have the link, but if you search you will find it quickly.
I won’t be building it into my frame immediately, but setting myself up for phase 2, to add a 6-8” wide skirt around the perimeter of the 4’x8’ work piece, to support the sled when cutting on the entire surface. If this is of interest to you, consider how you would attach it to your frame now.