I have a raised screened-in porch off the back of my house that creates a sort of car-port. I currently park my truck under there. I want to close in the area to make it an enclosed garage.

The porch is supported by concrete pilings and post/beam construction. The ground is currently compacted gravel, and I want to pour a slab there to make moving equipment easier. The concrete footers extend about 6-8 inches prood of the gravel.

Some things I'm wondering before calling a concrete contractor.

How thick of a slab does a standard residential garage need?
Will the slab interfere at all with the pilings supporting the porch?
Part of the area is held back/level by a timber retaining wall. It's very solid now, but will the weight of a slab be too much for this?

Any ideas appreciated.

You will likely be vastly increasing the load on the timber retaining wall (possibly 10+ tons added with concrete slab). That's the first concern I see. How high is the wall?
I would want a 6 inch concrete slab with required footings (rebars both ways in slab bottom half, and mesh to resist cracking near top). Check building codes.

I can't picture this entirely, but won't the footings needed to carry the slab and walls (3 walls I assume) negate the need for the present vertical porch supports via the concrete pedestals, i.e. the garage walls will support the porch). Sounds to me like a reinforced concrete retaining wall will be needed.

A photo would be very helpful here I think and I think that you need a contractor with design capability and/or an architect/engineer to look at your options and to ease the permit process.

I was thinking of leaving the porch support beams and just hanging curtain walls to close in the parking area, but now that seems like it's not the best approach.

I guess this is one of those things that appeared quick & easy at first but will turn into a major project once fully vetted.

Maybe you can do something like you are suggesting. Run it by a good contractor (or talk to your permit dept. or both).

This is just from a sample city specification but discusses enclosing a carport:
"Carports:
Enclosure of more than two sides will be considered a garage and must meet the code requirements for a garage.
Walls enclosing carports must be supported on a continuous frost-protected foundation that is at least 36” deep
below the ground or be totally unsupported by the slab."

Requirements in your area may be entirely different.

And instead of adding all that weight with a concrete slab, could you add 2" or so of a "crusher run" type stone material to top off the gravel and make it smoother.
"Crusher Run
Crusher Run is made from crushed stone that range in size from very fine (often called "stone dust" or "fines") to medium sized pieces. Due to the angularity of the pieces, crusher run compacts well, making it an ideal product to use as a base for pavers, as well as a wide range of pavement surface materials. To achieve maximum compaction, crusher run should be spread to a depth of 2-3", then wetted, and compacted with a vibratory plate tamper/compacter. This process should be repeated until the desired depth is reached. The result of this wetting and tamping process is that you will have a base that is similar to concrete, except that water can flow through it making it less likely to heave during the many freeze/thaw cycles our area is subjected to. "