If you're talking about the three small beams at the top, I'm not sure if FreeCAD will help you figure out how to size them. It will depend on two things:
1) The material that the beams are made of
2) The amount of weight they are expected to hold, which is effectively a third the weight of the large front sheet.
You should be able to calculate the weight of the large front sheet, as long as you know the material. Once you know the material, you can look up the density of the material, then multiply it by the volume of the sheet to get the approximate weight. Make sure you keep track of the proper units!
Once you know the weight, you can use typical beam equations - or online calculators - to figure out if your beams are beefy enough.
Here is one resource. If you scroll down the table, you'll want to look at "Cantilevered beam with one load applied at the end". The value of the load is one-third the weight of your sheet. Then, you can play with the beam geometry until you are satisfied that it is "strong enough".
I usually design things with a factor of safety between 1.5 and 3. I'll go up to 3 if it is not too cost prohibitive. A factor of safety of 3 in this case would effectively mean that one beam could hold the entire weight of the sheet on its own without deflecting "too much". It's up to you to decide what "too much" is, though you'd probably want to keep it inside the range of plastic deformation...
