Size Steel Trapezoid Frame

Looks like you have a 3-dimensional frame here.

How will it be loaded?

Can you at least show some drawings/sketches?

I interpret what you are saying is that this is actually pyramidal in shape. If looked at in plan view, the top and bottom are squares. The four sides are trapezoids that lean in.

How is the load applied? Depending on that I would start out designing by figuring out how the load is distributed at the joints using statics (vectors using x, y, and z componets - remember all the forces in the joints have to sum up to zero). Free body diagrams still work great. The top members will be the most "difficult" to design because, depending how the load is applied, they will likely act as beams with a column load of compression. If I have interpreted this shape properly, the four pieces that make up the sides would be designed as columns. The four bottom pieces would likley be designed as tension members.

After you have completed the loading at your joints for the static condition, add your pushing force and friction forces of the wheels as loads at the joints and reanalyze your free body diagrams. It is usually easier at this point, because now you have model to work with.

After you have the load for each member, then design each member appropriately.

If you don't know how to do that, then this discussion site is not going to help you. But there may be an engineer who reads this site who would be happy to do this work for you. I think you should first wrestle with coming up with the loadings.